because it annoys the heck outta most people and some people just live for that sort of thing.  me included.  nothing i like more than to see the "buzzkill" tag come onto the screen when i hit some unsuspecting runner/gunner with his little akimbos out and know that he just missed his killstreak at the hands of this camper.  I love to get the hate messages immediately after a game from some guy i killed, ruining his MOAB bid.