So would there be an uproar if they cast a 100% Chinese dude instead of the Ozzy chap? Thought not.
Just googled Billy Sing pictures and he looks more caucasian than asian to me.
As long as the film does him justice and portrays him accurately, I can't see what the fuss is about.
I would be more concerened if Mel Gibson was directing it 'cos we all know how he likes to remake history with his films.