That's because it's not funny to spawn in and expect a fair match, only to be headshotted from the other side of the ****ing map by a guy with a machine gun or pistol. No skill involved at all, no sportsmanship, and really you're not even playing the game. It's pretty much a big middle finger to all the people who've worked hard to develop their skill in a game, only to be put in an impossible match against a cheater.
No, using a different controller is not cheating per se. It is however unfair because no matter how much the other players practice (i.e improve their skill levels) in this case they're never going to be able to fire as fast as you can. I.e you have an unfair advantage over them, not because of your skill but because of your hardware. They have hardware which is perfectly adequate to run the game smoothly and well, but you've actively gone out and bought something which is designed to make it easier for you (less challenging) to beat other players. Only noobs would need to do that, and only noobs do. I don't mind someone who's just learning the game, but someone who's essentially given up on developing their skills and turned to other methods that make for a slanted playing field...well on any server I'm on you won't last too long before being kicked.
As another example for what I mean by a fair match, if you had a ping of 20 and your opponent had a ping of 690 you wouldn't say it was a fair match. You're not cheating, but that doesn't suddenly mean the match is at all fair. In fact I'd find it plain unenjoyable because no matter how good the player with 690 ping is, they'd almost certainly be pwned even by someone who's new to the game.