Firstly, what things are considered socially acceptable are not necessarily logical, and are tied up to historical accidents of how that culture evolved. For example whether a church should always be treated as a place of solemn quiet, or could also be a place of loud laughter and celebration varies from community to community.

In this case it's pretty obvious what the history is. The N word was used back in time by white people in a way that was hostile and belittling of black people, in circumstances where they were often seen and treated as sub-human. I guess maybe black people took to using the word ironically at first, but nowadays a lot seem to use it as a term of familiarity and affection.

Anyway, where it's seen as acceptable, it's seen as acceptable because there's no doubt that it's not meant it a racist or nasty way. The same way that Jewish comedians can make certain jokes about Jewishness, but it might be unacceptable for anyone else to do it.

Now some people think the N-word *shouldn't* be used by anyone, black or white. But that's a different story to why it's considered acceptable or not.

Read this interview about maybe the most famous example of its use:

http://bennun.biz/interviews/chrisrock.html