Well, when I go out to the woods, pitch a tent, set up a fire, and, oooh oh oh.. right.
I used to play a lot of Counter Strike Source. Sometimes camping is integrated into the game, but sometimes it's not. I've always considered camping to be idling in some nook waiting for someone to happen by, and blind siding that person. The caveat being, this is not camping if it is protecting the objective. Hidden along the path which happens to be the only way in for the opposing team to complete their objective, for example.
I've been called a camper a lot just by doing the objective of the map. I could be a terrorist protecting the hostages; right next to hosties in fact, and take people out. Next round, I'm assaulted by colorful language calling me a camper.
Camping is legit as long as it incorporates the game/map objective. If a terrorist needs to plant a bomb, and he's crouching in a far off corner while the rest of his mates are dead, and the other team has to start hunting him down, that is unacceptable camping.