My understanding of the issue is that anyone can engage in a race based action, but white people are incapable of being victims of actual racism. That's because a black person guilty of committing an act solely based on race (black actors painting their faces white) does not result in racism, which would be systemic oppression of white people. Racism is about power, and blackface is at the root of much of the institutionalized racism that exists today. That is to say, these actors painting their faces white will never result in white people being oppressed, but blackface was literally used to dehumanize black people, and it worked. Without power a minority group can never install a system of oppressive institutionalized racism on the majority, thus they can never do anything that would lead to systemic racism. But members of a minority population can do things that could be considered a racist act in the sense that it was done solely because of skin color.

tl;dr: As I understand it racism is about power, and institutionalized racism requires the power only white people have. White people can be a victim of a race related action/crime/etc., but white people cannot be victims of actual racism.