The objection in this case would be from the left. Historically, there's been great prejudice against nonwhite actors in Hollywood; they've been systematically excluded over the decades, either by having nonwhite characters rewritten as white or having them played by white actors in makeup. So while recasting a white character as nonwhite, as with Heimdall in Thor or Perry White in the upcoming Man of Steel, is seen as a progressive move, a correction of decades of discrimination. That's something the left would be fine with but that provokes outrage from people uncomfortable with racial diversity. Conversely, casting a white actress as a character who's always been black would be seen as perpetuating that tradition of discrimination and would outrage the left.
Remember, even today we're still getting nonwhite actors excluded from roles they should be getting -- the main characters in The Last Airbender and Starship Troopers, Ra's al Ghul and Bane in the Batman movies, etc. There's still a lot of prejudice, a lot of executives who assume movies with nonwhite leads can't make a profit. We're far from achieving sufficient racial neutrality in Hollywood that casting white-for-black and black-for-white can be treated as equal and interchangeable choices. It would be great if we were at such a point, but we're nowhere near it.
