I think while discrimination does definitely exist on both sides the separation between the two in America stems more from cultural differences than from out right bigotry. I say this because I think that the argument, which says there is an Asian-American Buddhism versus a white American Buddhism in the U.S. is too simplistic and dualistic.
It misses the important point that the term "Asian" includes dozens of very different cultures/countries. This is evident here in America because there are Asian-American Buddhists who separate amongst themselves even.
Korean American Buddhist communities often stick to mostly Korean American Buddhist sanghas, Vietnamese with Vietnamese, etc. Of course that's not always the case and there are examples of well integrated sanghas mostly in highly diverse urban areas of the country. Often white dominated sanghas are such for no other reason than the sanghas location being in an area where whites represent the majority population in general.
Besides, I don't think that we can completely uproot cultural differences within a greater Buddhism nor is it necessarily a bad thing to have different cultures represented within Buddhism. There is much beauty and value when culture meets the Dharma but like with all things it can be a hinderance at times too. So yes, again as I stated above there are instances of discrimination on both sides but the beauty of Buddhism is that we all share the core beliefs. Dharma is Dharma whether it is spoken in English, Thai or Norwegian.
~Peace to all beings~