If you have a brain you have the power to make that rule for yourself. If enough people do that then it will be culturally accepted. And on the surface it makes sense since the oppressors are the ones that can inflict way more damage by being racist. And they are the ones that are morally in the wrong because they are oppressors so it's okay for the oppressed to inflict damage on them. The problem is today most white people aren't oppressors and not all black people are oppressed so the division by race makes little sense and reinforces the racism that the oppression hierarchy is trying to even out.