I did join in on the discussion. You just didn't respect my opinion.
In PoP's case, they are being faithful to the game. In Avatar's case, it has to do with a) it's a kids show and not an historically factual one, b) only people outside the target demo seem to care, c) it's about casting actors that can play a character.
And finally, there's the whole commercial reason, which from a business point you have to be sympathetic with. You can't go out of your way, risk not getting the best actor for the job, risk casting actors people don't want to see, just because 5% of the audience of the movie feels that Hollywood has to make a 150 million dollar budget movie with a cast made out of Hindu people.