I am guessing they are sick of police practices like this:
https://www.washingtonpost.com/news/...=.8ab540651503
The shooting are the extreme cases and may be few and far between, but the everyday harassment isn't. Quick anecdotal story from me. I've been pulled over probably half a dozen or so times in my life. Every time for an obvious infraction, except one. On that occasion I was pulled over for "failure to signal", and I have no idea if it was legit or not. But I happened to be driving with a black friend, and after the cop asked for my docs, he then when ahead and asked for the ID of my passenger as well. Goes to the cruiser, comes back, has my buddy get out of the car, does a full pat down and starts giving him 20 questions. Meanwhile I am comfortably sitting in my car. Ends up that his name was supposedly similar to someone with outstanding warrants and we go on our way. Maybe it was a one off and had nothing to do with him being black, but there sure seems to be a lot of stories like that. Ever had a cop check the ID of your passengers for a traffic violation? Ever been quizzed because your name is similar to someone else's? Ever been taken out of a car and given a pat down because you were driving with someone who didn't use their turn signal? Those were all a one time thing for me.