Rozen's motivations are the subject of some debate. Some believe that his apparently uncaring attitude is indicative of an unkind person. Specifically, the paired facts that he has left the dolls alone to suffer whatever befalls them, and on top of this told them to fight each other to the death. The basic idea which runs through all anti-Rozen ideas is that he created the dolls to search for some ideal which he was pursuing, not caring what happened to them in the process. This is based on the simplistic idea that the mere fact that he allows them to suffer at all makes him unkind, when the actual situation is more complex.

The abandonment argument is invalidated by the fact that Rozen hasn't really abandoned them. When he deems it appropriate he steps in and modifies the situation. Two examples of this which we are given are when he saves Suigin Tou and gives her a Rosa Mystica, and when he returns the Rosa Mysticas taken away by Bara Suishou. The fact that he does not intervene the rest of the time does not mean he has actually abandoned them, simply that whatever the dolls are doing at that time are within the 'rules' he has set for their existence - these rules were originally believed to consist simply of the Alice game, but it is revealed at the end of Traumend that there is more to it. This actually strengthens the other argument against Rozen - that he puts the dolls through suffering for his own ends - as we would now count the entirity of the dolls lives, including everything bad they have ever experienced even outside the Alice Game, as part of what Rozen put them through in order to try to find Alice.

This other argument is more difficult to counter, as it is not technically wrong - the dolls were created to, among whatever else, fight each other. Why would he create them like this if he were not an unkind person? This touches on the same sort of fuzzy philosophical areas as asking why, if a caring god exists, he/she would make us mortal, able to feel pain, etc. It is simplified in the case of Rozen Maiden in that we know they have a specific purpose - becoming Alice. Even though Rozen eventually reveals to Shinku that they do not have to play the Alice Game, he still encourages her to try to become Alice in other ways. Since we know that the purpose of the dolls lives is to move towards becoming Alice, and we know that Rozen will step in when something goes outside the bounds of this objective, the lives they are leading were not simply dumped on them because Rozen didn't care; they must be in some way be part of the search for Alice.
This alone does not change anything, it still involves Rozen leaving them to live unhappy lives, even if it is for a greater purpose. However, we must also consider the context all this is happening in. One of the prominent themes in Rozen Maiden, especially in Series 1, is that everyone's life is hard in some way, but that it is preferable to continue to live nonetheless. Shinku says practically exactly this with regard to the Rozen Maidens near the end of Series 1. Jun comments that he believes that the dolls being created for the sole purpose of fighting each other is a sad story, and Shinku disagrees, saying 'to live is to fight' (and, of course, 'to fight is to live', because all the best catchphrases are reversable). Obviously in the case of real people this isn't always true literally, however Rozen Maiden has a habit of taking abstract ideas quite literally in the plot, so while the fighting Shinku refers to is actual combat for the dolls, what this is actually a reference to is the fact that everyones' lives involve conflict and struggle to some extent in some form. So to say that the Rozen Maiden's lives are sad because they have to fight is to say that everyones lives are automatically sad because there are bad parts to everyones' lives. You can make of that whatever you want, it's not really relevant to this, the point is simply that it pretty much ends the argument that Rozen is uncaring because the dolls he created have hard lives; it is, in the worldview of Rozen Maiden, simply the nature of life that there are bad elements, and the Alice Game is theirs. The only real alternative for Rozen would have been to not grant them proper lives in the first place.

This argument isn't enough to completely deny the possibility that Rozen doesn't actually care about the dolls, but it is impossible to be completely certain what kind of person he is. There is at least a lot of doubt around the hasty assumption that he doesn't care. Looking to the series for an answer, it suggests that he does actually care about the dolls. Rozen is always presented in a positive light by the dolls themselves when they mention him, it does not occur to them that the Alice Game could mean that he does not care about them. It because of a desire to return to him that some of them play the game. Of course, he made them, so he may have simply designed them to think that way. We are also shown brief clips of when Rozen was working on the dolls, specifically when he was making Shinku and when he fixed some of the dolls at the end of Traumend. In these instances he does not act as if the dolls are simply means to an end, he acts as if he genuinely cares about them.
Enju's role in Traumend provides contrast for Rozen's attitude towards the dolls. For most of Traumend the viewer is intended to believe that Enju is Rozen, and that this 'Rozen' is indeed far more concerned with the Alice Game than with the dolls. The truth of the matter of course is that Enju is only concerned with his own doll, and only wants to see the true Rozen Maidens defeated, to prove that he is the better dollmaker. The cold attitude he has towards Rozen's dolls contrasts with the way Rozen acts towards them the few times they are seen together, which is the point; it seems for a time that Rozen doesn't care about the dolls, but then it turns out that that isn't really Rozen. Enju also proves that Rozen has no need to pretend to be kind to them; Enju got them to fight simply by ordering them to do so, so any ideas about Rozen only pretending to care in order to keep them fighting are invalidated. A final point with Enju is that despite not caring in the slightest about Rozen's dolls he does care about his own one, but he sends her off to fight as quickly as he does Rozen's dolls. So in the case of the dollmakers it would seem that wishing them to prove themselves in the Alice Game and being concerned for them are not mutually exclusive, an idea I already discussed above.

Moving on from whether or not Rozen cares about the dolls, and assuming for the moment that he does, the next question is why in this case does he keep his distance and allow them to suffer at all? I have already answered this above, that in order for them to live proper lives they must be allowed to live. The point I wish to make here is that we know that Rozens motivation behind creating the dolls and the actions he takes regarding them is the pursuit of Alice. Therefore it makes sense that reason he decides to let them live proper lives and not intervene is that this is necessary in order to find Alice. This ties in with Rozen telling Shinku directly that there is a way to become Alice besides the Alice Game.
Now we need to consider what this search for Alice is really all about. As I pointed out before, it is originally presented simply as a magical construct within the series involving the magic used to create the dolls. The fact that Rozen leaves the dolls to their own devices whether they choose to fight or not, however, suggests that it also has something to do with the dolls living their own lives, rather than being protected from life by their creator, and so it starts to step into the area of the overall themes of Series 1. This pursuit of perfection, magical doll specifics aside, is the pursuit of bettering yourself. This appears to be what Rozen is getting at in letting the dolls live their own lives. The idea presented in Series 1 that life in worth living despite the hardships and the path to becoming Alice actually seem to be one and the same. This idea can even be related to the completely fictional magical aspects. It is shown in Series 1 that how useful a medium Jun is is related to his 'strength as a person'. Jun becomes a more powerful medium by doing what Rozen appears to be encouraging the dolls to do, so even the magical elements of the story tie into this idea. Literal perfection itself cannot be reached by real people of course, but it is pointed out at least a couple of times in the series that significant difference the dolls and humans is that they can reach perfection, so the difference is simply thats this journey of self-improvment actually has a final destination for them.
This idea nicely ties the various themes and elements of the series together, makes the aspect of Rozen Maiden which was probably least applicable to the real world, the pursuit of the perfect girl, tie into the parts which are more applicable, all the stuff about living life being good etc., and it explains how Rozen, despite caring about the dolls, would leave them alone to live their lives without him.