Has nothing to do with race it's about changing a established concept, like casting Britney Speas to play Gandalf. Understand?
Sure its about race. Will Smith is a good actor, and a solid box office draw, but people relate to Dirty Harry as Clint Eastwood (who is a white man)
You can get away with casting the next Bond as a black man, because you can just claim he is a new 007 agent.
It's like casting a black Batman, Superman, or remaking John Wayne movies with a black guy.
Whats next, getting a white dude to play Shaft?