I think it's because some of the campers I see can't be enjoying themselves.
Well personally I don't load up a game to sit at the end of the corridor, aiming down it for 10 minutes just for one person to wander past.
Yeah, this is only a handful of campers, but it's the worst examples that stick with me. And probably with everyone else. Hence the camping hate.
EDIT:
Oh, also, campers on objective games can get annoying. Holding a position because we need to defend, that's fine. But when the entire team just sits around their objective and makes no attempt to go for the enemy's, it can get a little... tiresome.