The US has had a series of systems for oppressing blacks: slavery, segregation laws, and now repression by thugs leading to mass imprisonment.

The victims don't have to be limited to blacks. Rather, when economic pressure is the motive force, it catches whatever groups are easiest to catch. That often tends to be the groups that are already in a weak position, perhaps because of lingering effects of an earlier system of oppression.

To the extent that bigotry is practiced (consciously or not) by thugs, the target group must be one they can recognize. But that doesn't mean it has to be a racial group. It can be the poor, if they can be recognized as a group by how they dress and where they are found.