So blacks can say 'nigga' but whites can't? Was kinda under the impression 'nigga' was a reclaimed version of '******' and more a term of endearment now than a slur. And when it's a parent speaking of her kid, especially her white kid calling him, 'disnigga' I'd think it's probably not a slur. Have heard it used in many movies from non-blacks using it as a term of endearment for black friends so I think this is more an anti-Madonna thing, than genuine outrage over a supposed slur.
We need to start assessing context more and not decrying every instance a given word is used as a slur. Boy Who Cried Wolf being the reason. If every instance becomes a slur, we'll begin to tune out such usage just as we have profanity. Profanity's so common now it's no longer 'strong' language as people tune it out. If every instance 'nigga' is uttered is a slur, we'll tune that out too and then when it used as a derogatory or slur no one will care.