People camp usually because they're shit, you go toe to toe with them, they're dead, but thats no excuse for sitting in a corner and shooting people in the back that don't have a clue you're there, it's just not very fair. I don't know how this subject come up, it's usually me moaning about them, but I've not had too many problems lately as i've been playing barebones pure which strips away the killstreaks, perks, attachments etc, so theres no reason for people to camp, although you still get the odd goon or two.
I hope the TAR-21 is in MW3, been using that on MW2, beast of a weapon.