I don't like seeing this kind of parody either, since the Olsen twins aren't yet 18. Plus, they haven't gotten in trouble with the law, which I think makes celebrities fair game for humor. (I don't include people like Jenna Bush in my definition of celebritydom.)
Here's why I think someone chose the Olsens for parody:
1. They have a squeaky-clean image, which brings out the cynics who believe that it's all a pose. The cynics believe that everyone has skeletons in the closet.
2. Some guys have a certain unhealthy fascination with watching young girl actors grow into maturity (Alyssa Milano, Christina Ricci). I find that distasteful, not just because I have daughters, but also because that's like lusting after your sister or cousin.
Sign at a hockey game: "500 Days Until Mary Kate and Ashley are Legal" Sure, like their first boyfriend would be some drunken shirtless guy who acts like an idiot in public.