I would venture a guess that they likely have already gotten a couple of complaints about the content of the show in relation to the age of the actors, and in order to prevent them from giving ammo to the complainers that could be used against them, they can just remove any content that might expose them to potential legal action. It may be relatively innocent, but if removing a young person's butt from an episode keeps the government from taking action, then it's well worth it for the company, especially when you consider the amount of money they would spend on fines and legal fees in the event they had to defend themselves against some charges.