Yeah, like I said, I was mistaken in referring to Katara and Sokka as Asian and corrected myself. But that doesn't change the issue at all. Non-whites lost roles to white people and a rationale that says "Well, this is clearly a fantasy setting so everyone is without ethnicity" is absolutely racist. Not "racial slur" racist, but the product of a de-facto racist institution.
If you truly want to understand why this casting is particularly upsetting, start by visiting the blog that rockmanj linked to earlier:
http://www.racebending.com/v3/featur...rimer/#casting