They get away with it because they aren't doing anything wrong, necessarily. With the exception of adult only content, there aren't laws requiring ratings on websites in the same manner as, say, movies and TV -- and while this is suggestive, it doesn't quite hit the AO mark. Further, I haven't seen the picture in context of the full flash and website it's on, but in the original post, rover said
My concern is that it is inside a free flash games that to me is aimed at kids.
-- 'to me is aimed at kids' -- which makes me think it seems like a childrens game because of its simplicity and cute rodents, but is perhaps not explicitly aimed at kids.
I'd wager it's just a blacksheep.com ha ha shock value deal, maybe designed to seem like it's for kids while being adult themed (ala peewee herman's playhouse