"Aim" just means you're not shooting from the hip.
It's like using Left Trigger to Zoom when you're playing shooters on your xbox. It doesn't mean you're going for a headshot.
Though you make a good point, most militaries train you to aim for centre-mass, which in a sense is a Called Shot to the body, Their motivation for doing so, though, is not reflected in the rules. In RL, aiming for Centre Mass increases your chance of a hit. In game terms, if you miss the Body, you are likely to hit a limb or the head. In game terms, this is not true. Miss a Called Shot to the Body, and you basically miss EVERYTHING.