Probably because a lot of the kids who are heavily into the group are... well, douches.
Personally, I don't hate the band, I simply don't listen to it. But the few people I've met who actually do listen to it and get into the whole thing are people I wouldn't choose to talk to again. Definitely not my crowd. Any hate is probably from people wondering why something that appears so silly and trashy gets taken so seriously by some, even going so far as taking a group name for the fandom. It makes no sense to me either, to be honest.