There's a fundamental difference between a racial or cultural epithet and a word like "Nazi". The former insults a person because of who he is. Nobody has any control over the color of his skin, where he was born, or who his parents happen to be. Words like "Nazi" insult a person's attitudes and behaviors, things over which he does have control.

If you're a vegetarian who attends a party where meat is served, and you harangue the host and other guests about it, someone is probably going to call you a "food Nazi". It's an insult, but if you don't want to be insulted all you have to do is keep your mouth shut.