The issue whenever you address this sort of thing is that there are two definitions of "racism" floating around in the discourse:

(1) Racism as an act, e.g. someone saying to me "you are worthless because you are Asian."

(2) Racism as a structural and systemic phenomenon, e.g. the fact that throughout the late 1800s and early 1900s, black people in America were purposely prevented from purchasing certain kinds of property in order to produce all-white neighborhoods. This sort of thing has a lingering effect, namely in the production of ghettos, etc. Hence, we call this problem a "structural" one.

Both definitions are correct, but which one seems more "proper" depends on the demographic, and favoring one over the other seems to leave something out of the conversation.

Those who say that minorities cannot be racist against white people are simply incorrect if they are using the first definition. People commit horrible acts on the basis of other people's race regardless of their own race. Just the other day a friend of mine who is Asian said, "That person is stupid because she is white." That would be an overt act of racism.

However, to simply reduce "racism" to individual acts is just absurd. The example I cited above is just one example, in which there is a structural prejudice against certain minorities. The mistake a lot of people make here is that when they hear the word "structural," they think it means "a group of individuals." However, again, that would reduce the problem to an individual one. Rather, the point about calling it a "structural" issue is to say that it has deeply embedded effects in economic, political, social, and even academic structures such that, as participants in those structures, we will become affected in ways other than overt, willed racism.

When people say that minorities can't be racist against white people, if they are using the first definition that is simply incorrect. However, if they are using the second definition, insofar as there have not been (or, if there have been, they are so few in number) structural prejudices against white people in American history, they seem to be onto something. Nonetheless, I would probably say that the idea should be phrased better.

That's why I hate this sort of question: it ignores so many of the underlying issues that are involved and approaches the whole matter the way a five-year-old would.