"Mostly"
-
Interesting that the conceit of the joke is the argument that the protesters are to be properly judged by their worst contingent. But we apply the opposite to police officers, who are judged only by the lawful majority. It's pretty hypocritical.
My guess is that the people who do that relate more to their idea of police officers as a group than they do their idea of who protesting black folk might be, so it's bias based on a mix of low-resolution guesswork about the two groups and the limitations of personal experience.