Eish dude. That sucks. Is a big reason why I don't play MW2 no more. The game itself is lekker, but the non dedi servers and thus lack of any kind of control is a MASSIVE turn off.
As for why. Well, why do people murder, rape, steal etc? Cause they are fecktards. Some people take enjoyment from spoiling otherss' fun. Sad, but true. It's the same principle as taking steroids or whatever to be faster, stronger etc. The day I can't keep up anymore and need to hack to keep up or enjoy a game is the day i stop.