There have been multiple posts here in /r/books about casting black actors in adaptations of popular books and every time there are multiple comments bemoaning the fact that it's a black actor being considered.

If you're here complaining that the actor cast is a different race than how you perceived the character in the book than you need to stop. Unless a character's race is actually important to the story, and neither Hermione's or Roland's is, then you're just being racist and you need to reassess your views of the world.

Also, adaptations are not meant to be carbon copies of their source material; parts will change, will have to change, when adapting something to TV or movies. Otherwise, movies such as The Hustler, Jaws, and The Godfather would not exist in their current; we also would not have Morgan Freeman in Shawshank Redemption.