It's based on some studies that show some correlation between teenagers being more aggressive when playing violent games, done by psychologists and whatnot. It's not a matter of prohibition, but just to discourage it.

The actual psychologist's conclusion is that if the parents keep a good relationship with said teens, that is more likely to keep their aggressiveness down, since there's no actual way to link games to violence directly. So this is more along the lines of taxing cigarettes and alcohol to make it so kids don't isolate themselves or grow addicted, as well (or so they claim).

Also, the tax is 8%