That's really the whole point, IMO. They don't. We do. If you don't like it, don't go and see it. I'm pretty sure that the tone and content of this film (and most, if not all, widely released films) was well enough publicised. If, however, someone wants to go into a film cold, without knowing anything about it, then they risk being offended. That is also our choice.
I am not easily offended by films. The only ones I can think of are Henry: Portrait of a Serial Killer and Sŕlo. People underestimate the power of the movie experience. HK, you say that the makers of Kick-Ass do not draw the line, but that it is us that draw it. Imo, a filmmaker has the duty to draw the line somewhere, and when you show a kid getting beaten up by an adult under the pretence of it being only a satire, that is crossing the line.
Your argument of "if you don't like it, don't see it" is invalid imo, because if that's the case, then filmmakers can just show everything in movies, as long as it's accepted by whatever committee that determines the film's rating. So that would mean that I can make my satirical underage sex movie and justify it by saying: well, if you don't like it, don't see it! That would be all too easy.
Coincidentally, I've just read a quote by Peter Jackson that Mark put on the forum: "Film is such a powerful medium. It's like a weapon and I think you have a duty to self-censor." He said this of his own movie The Lovely Bones, but it sums up the issue I have with Kick-Ass.
This may not be a popular opinion that I'm putting forward here, but I've never backed off from voicing what I believe in. Until today I had only read a few Belgian reviews on Kick-Ass, which were all positive, so maybe I am being an old-fashioned moral crusader here. But I don't mind as I've got this man on my side .