Because the ESRB are idiots. They give out their highest rating for the game, and still limit the game in terms of censorship. I'm pretty sure i won't be walking outside and blasting kids heads off with a laser rifle in vats anytime soon, its a shame PEGI think i would. Nevermind.
Further to which Germany and the UK will ban games outright, Germany because of Nazism, the UK generally if a previous game in the serious is claimed as causing a crime. I fail to see how else Manhunt 2 was banned but Condemned 2 got through.
EDIT: I mean in case of ingame Nazism, not the Nazis themselves ban them.