I think perhaps the 'n' word being used by African-Americans is similar to the word ''queer'' being used by gay people. Both words began as a disgusting insulting word used by racists/homophobes. But they got adopted by black/gay people, almost as a way of ridiculing the word and making it less of an insult. Almost as a way of taking away the power of homophobes and racists and saying ''F**k you, we've hijacked your word and make it something positive so you can't hurt us with it.''
I'm not saying I agree with it, but I can see where people are coming from when they use it.