I see what you mean, although your example makes me feel uncomfortable because I think the word "retard" used as an insult is much, much more offensive than the word "canon nazi" and the sort of stuff one really shouldn't be using.

I understand how generally speaking people trying to tell other people what they should or shouldn't say can be irritating or feel stifling; on the other hand my experience with what you would call the "PC movement" have been much more varied. There's a lot of different opinions on what is or isn't okay, and a lot of discussions and sometimes arguments in order to reach any kind of consensus; and while I've seen dogpiling happen in cases when I didn't think it was warranted, in the majority of cases I've seen, it occurred because the person being dogpiled was the one reacting in a very aggressive and frequently insulting way in answer to what was first a polite request.