I think it has to do with the prominence they take in the story. If the protagonist is black, having some sort of magical gift is a lot different than having a barely verbal, giant black guy who appeared out of no where and started healing white folks who get to kill him as a favour. It has the surface appearance of being anti-racist, but in reality is sort of the opposite.
ETA: I didn't read the book, so there might be more to the character in it, but really, John Coffey is more of a sketch of a character in the film. He does nothing but sleep, talk in a slow, dim witted way, and help white people including the main character. It's like he is a prop rather than a real person.
I rewatched The Stand a few weeks ago, and the portrayal of Mother Abigail made me extremely uncomfortable. I remember watching it when it first came on and thinking it was a little cheesey, but now, I found it really racist.[edit2-plus the lack of diversity in the survivors...that was a bit shocking]. It's funny to see how our awareness to things like that evolves Don't get me wrong. I like Stephen King. I just really don't like some of his crutches.
