They are role playing mechanics - the games just model that a person's impulse responses may not match their ideals. The reason the players are removed from the PC response is because there are defined mechanics that are checked to determine behaviour in certain circumstances. "I'd never steal money!" "Well, there's a bag of unattended money in front of you..." "I shouldn't have taken the $30,000! Now what do I do!!?!?"
Champions/Hero System has a related set of mechanics where the player defines specific personality issues for a PC and is constrained to play within them. The more extreme the reaction, the less player control can be exercised when they are triggered.
Players need not have complete control of their PC on a role playing game. Indeed, if there are any mechanical systems for social interaction or strong emotion then almost by definition the players cannot have complete control.