The auto-generated images are free. In this event, to prevent people from flooding Roblox's image moderation queue it was necessary for them to set a limit. It's rather disappointing, however, seeing auto generated images entering moderation queues, but I can somewhat understand Roblox's stance on the issue.
As before with the auto generated images, a user may make a rather inappropriate place and the thumbnail clearly presents that. In fact, this exact issue occurred on mother's day (of 2014 I believe it was). Since the good chunk of the people that would play those games were young immature boys, those games made it to the front page rather quickly. As many can tell Roblox's moderation team did not appreciate this and took them down. Personally, though, I believe Roblox should have implemented the auto generated image moderation within days of the event but some other event may have lead to the institution of this feature.
From a parent's perspective, the website might be seen as a place with relaxed guidelines on user creation and try to prevent their younger children from playing. Additionally, parents, people in general, want to see what they are paying for. If a parent were to go to the front games page and, like the issue I just described, saw the content of that nature, they likely would keep their child off of Roblox and not purchase Builders Club or Robux.
There also comes into account the legal aspects as "adult content" isn't supposed to be easily accessible to minors. For a website designed for kids, there is that chance of legal issues and the possibility of lawsuits.
The topic may go more in depth and there may be other reasons behind the regulation of auto generated images but these are some key points I believe seem logical. I am not a Roblox employee myself and I certainly am not speaking on behalf of the corporation. I am simply expressing my insight on the topic and welcome anyone to provide their opinions on the matter.