Political Correctness is a form of passive agression. The entire premise is to pass it off as 'politeness'. It works in a certain way because its a defense mechanism for people entrenched in power an privledge. If you don't get the irony yet, keep thinking. It will come.
Unfortunately many people misuse that term when they mean "politeness" or similar. These people don't want to stop using the word retard wherever they want, and they want to be outraged when people point out that they're being assholes.