Dudes just say that so they don't upset any white people who like their music or associate with them. It takes a lot more guts to be honest and challenge white supremacy than it does to say you don't care if whites, Hispanics, Asians etc call you a nikka. Africans be scared as shyt to call people out on their racism.
My brother was fukking with this white lady in her 40s and he was letting her say nikka just because she was rich and bought him clothes and shoes. I had to step in and check her because he was too worried about offending her.
In my experience I've found that a lot of times they'll say it real low around you just to see how you react. If you don't say anything they keep going. if you call them out on it they'll react one of two ways: belligerently or with the 'my bad' response.
Right now, with rappers like ScHoolboy Q, Quan, Styles P, and Tyler putting their stamp of approval on whites, Asians, Hispanics etc calling them a 'nikka' the white supremacists are conditioning us to accept a white rapper who says 'nikka' on his tracks. Hopefully by the time that happens we aren't totally brainwashed into accepting it.