It's not that you can't express racism if you're a minority, it's that it doesn't matter, because you have very little power. White people are by far the most powerful group in the US, so the systemic racism that is incorporated in to our culture has a real impact on people's lives. What impact does some black dude calling you a cracker have? Oh no he hurt your feelers. Grow a pair. On the other hand what does growing up with black skin do in a culture that systematically promotes black people less, is far more likely to put them in to prison, etc? The point is the impact of different people doing it is simply not the same.
People don't care about someone shouting black power because there isn't much of it.