The game Nazi Zombies is not a form of discrimination. A video game can not have discrimination in it. There are plenty of other games that have you fight other races and groups, and those are not considered having discrimination because it is fake. If you think on that line, almost anything you do is discrimination.
If we say that killing Nazis in a video game is bad, should we not be allowed to kill any race? Any bug? Any creature at all? Should we not be allowed to drive a toyota because it makes fun of Ford? No, they are CHARACTERS IN A GAME! They are a real group of people, but in a fictional scenario. Just because we kill a russian in call of duty doesn't turn us into a bunch of discriminating racist.