Why did many Avatar fans act to stupid over the casting choices for the movie?

I don't know why it was such a big deal. I've only seen a couple episodes of the tv show, and I thought that Aang looked Asian (but the kid playing him in the live action looked just like him, so what's the big deal?). I thought the fire prince looked Japanese. And I thought the water benders were either of Indian descent or Native American descent. I mean, it's a fantasy world so no one in it is Asian, or Native American, or Japanese to begin with.