I guess I'll play devil's advocate here.
I would assume a good portion of the people who downvoted the mod in that particular thread don't disagree with "don't use offensive slurs", they probably disagree with the idea that "retarded" is an offensive slur. The meaning of the word "retard" is so far divorced from its origin as a technical term for the disabled at this point that for the vast majority of young people (who make up the audience for this board) it has little to do with the disabled. "Retarded", as an adjective or verb, is even farther divorced from being a slur for a specific group of people. Its certainly still insulting to call something "retarded", but its not really a slur at this point, just another regular insult, and insults have value.
Now, I don't actually think its important to defend the right to use that particular word on here - as you said, its unnecessary. I do, however, think its a massive generalization of the people who downvoted that post to imply that they would like to use offensive slurs freely, and a frankly insulting one at that. Some of the people might believe in the kind of argument against "retarded" being offensive like the one I laid out. Some might believe that its offensive but not want it censored anyway, and would rather community reaction be the punishment than moderation. Some might disagree with moderation in principle. And yeah, some might just want to call people "retards". But definitely not all of them, probably not even a majority.