From other places this is experienced as craziness. What is "equal and ethical" about black women playing King of Sweden from IX century (this is a contrived example as I haven't even attempted to watch any of the contemporary historical movies from USA)? He wasn't black, he wasn't woman, why violate the history?
Especially, considering that most of the world has nothing to do with the slave trade in the US, or with genocide of Indians in the US, or any consequences of it. So why we should suffer raping of the history just so that Americans could "restore the balance"?.. Besides, I don't even think that it restores the balance. It's a superficial measure that is very cheap compared to restoring the equality indeed.