Your correct that watching violence doesnt make children crazy. However, when a kid (8 or under) sees someone kicking the crap out of people and IS NOT PUNISHED or worse still if they seen to be REWARDED, they will copy that persons behaviour in real life.
This is where the LEARNING comes in.
If the parent approves (laughs or cheers on etc) or ignores the kid running around the place karate kicking everything in site or pretending to shoot things, then they will see this new behaviour as acceptable, even as something positive. However, if they get shouted at or hit for kicking over things, then they will see it as bad behaviour and stop doing it (unless for attention seeking but that doesnt come into this).
In real life a kid can see the consequences of actions, for example if he sees someone gunned down in the street, he will see all the screaming, crying and the guy (hopefully) being arrested for it. You rarely see consequences like these in TV/video games, therefore it is the parental responsibility to fill in the gaps here, and reprimand the kid if he reenacts. In many cases where the kid gets out of hand, they parent has simply failed to do this.
And yes! I blame steam for violence in children.