EDIT: This is not meant to say that everyone thinks anime characters are white. However, there has been an overwhelming response in the Ghost in the Shell ScarJo is fine, because anime characters are white. As such, I thought it would be good to look at an article which does explain a bit of the sociological reasoning for this.

From Sociological images:

Why do the Japanese draw themselves as white? You see that especially in manga and anime.

As it turns out, that is an American opinion, not a Japanese one. The Japanese see anime characters as being Japanese. It is Americans who think they are white. Why? Because to them white is the Default Human Being.

f I draw a stick figure, most Americans will assume that it is a white man. Because to them that is the Default Human Being. For them to think it is a woman I have to add a dress or long hair; for Asian, I have to add slanted eyes; for black, I add kinky hair or brown skin. Etc.

Head over to this website to read more, and a fairly good description of why we tend to think this.