Here's the brief version: people who say "blacks can't be racist" do so, perhaps subconsciously, because "racist" has powerful emotional content, and they want exclusive use of that power to fight against oppression--so they can label others as "racist" (gaining that emotional power via language) but others can't use it on them even if they are "racially prejudiced".

They explain this as racism being necessarily about exactly the amount of power at exactly the institutional level that means that blacks "can't be racist" by definition. But I don't think this is the real reason.

I think the real reason is the desire to have a linguistic sword that you can live by but not die by, so to speak.

Is that clearer?