They do this because whites tip well and Chinese don't. It's pretty widespread and common knowledge, even though it's illegal so of course nobody will admit it, and not just restricted to Chinese restaurants - see the links.

In most non-Anglophone countries tipping is nonexistent to begin with, so foreigners get confused by this custom, and those who have been here a while can still be pretty cheap about this.

http://articles.nydailynews.com/2011-09-13/local/30170660_1_discrimination-federal-lawsuit-racial-slurs

http://gothamist.com/2011/09/13/indian_restaurant_accused_of.php