Let me explain my thought process through a video game analogy:
You're playing a competitive multiplayer game online (Modern Warfare 2, Team Fortress 2, whatever). You run around a corner and get shot and die without seeing where the person is. You watch the KillCam and realize that they are sitting on top of the building you just ran around. Fine, let's go and try and flush them out. You go and attempt to kill them, taking the same route and looking into the sky. You toss a grenade on the roof. Nothing. You assume they moved on. You turn away, catching a glimpse of the player as you turn. They react before you do, and you die and respawn again.
Now, at this point, the intelligent player would think to themselves, "Alright, well let's try and sneak up on them from a different way." So you go around the back way to there. Maybe you encounter (and possibly die from) a claymore. You either respawn and go back again, or you evade said claymore, sneak up on the player and knife them. Simple.
Basically, I don't understand why people complain about "camping". If you continually die from someone staying in the same spot the entire match, you're doing it wrong.
Also, snipers, by their nature, have to "camp". But you don't hear people complaining about that, now do you? As long as your gun has a scope on it, "camping" becomes a perfectly reasonably strategy. Hell, it's expected. So where's the line?
(Like everyone else, I think that "spawn-camping" is just bad play. But that just goes without saying.)