Hip Hop
Member
It's been trained into them by the insane popularity of COD4 and it's generation of shooters, you see a guy and immediately mash LT. This not only plants your feet so you don't even have to think about your own movement, but also gives the game a less obvious place to put in the auto aim as it will snap to during the FOV transition.
If you're talking about COD:
The "snap" only occurs in campaign. You still have to aim at the player in multiplayer.
and aiming down sight does not slow you down when using sub machine guns, they've also added the "Stock" attachment since Modern Warfare 3 that are optional to all weapons so you don't lose your speed when you aim down.