Now, I'm not usually one to draw the feminist card, but in this case, I'm going to. I think that (one of? part of?) the (underlying?) reason why so many people hate boybands is that they appeal to and are (largely) marketed towards young girls. As Silvia said, all bands are marketed, so why are the boybands catching the flack for it? My answer is, because they're marketed towards girls. Girls - or women in general, for that matter - are not supposed/allowed to have good musical taste. Any band or artist they like is automatically suspect, and if they like a band or artist that is generally liked by men, they're in it for the wrong reasons.
Women who don't play the male market (by playing on their sex appeal towards men) are often seen as whiny things, especially when their songs are about "women's issues" (quotation marks because I think rape is a universal issue, thank you; being a man does not absolve you from having to think/worry about rape). All-male bands appealing to a mostly-female audience? The horror! (Whereas bands like Limp Bizkit, who appeal primarily to a v. young male audience (and are thus as Silvia said a boyband for boys), get much less flack for it, because of course no matter how young they are, boys are allowed/expected to have musical taste.)
I'm not sure how much sense that made, but I'm hoping you got the gist of it.
From:
*nods*