Racism is a two-way phenomenon. Once upon a time, the crude categories of 'black people' and 'white people' was each acknowledged to be a group of different races that even have unfortunate rivalries against each other. In racism, sometimes there is a clear historical oppressor that wronged a minority, but the social strata doesn't reflect this. Not every 'white person' is to blame for the oppression of 'black people'. The need to retaliate fosters race wars at a social level, putting both sides at fault.
Society likes to find someone to blame, so typically the conflict is portrayed not in an accurate fashion, but in a just fashion, where the 'white people' become a cohesive enemy that is appropriately loathesome, and the minority of the day gets a hero's pass to break them down by any means necessary. Hence you get these comedians on TV who can say pretty much anything, and it's in response to this strawman 'white person' that is the personification of all the suffering and evil that plagues minorities everywhere, but it's heard by the actual people who know that they're considered 'white', making them go 'WTF, what did I do to deserve that?'