That's a big assumption. There are a couple things at play here.

One is the demonization of blacks through the way the war on drugs has been prosecuted. Blacks are far more likely to be arrested or incarcerated for drug crimes, despite an equivalent or in some cases lower incidence of drug use than whites. Blacks are more likely to be disciplined in school, regardless of behavior. This leads to an adversarial relationship and it creates what seems to be a trend of blacks always having run ins with the law (significantly because of unequal application of the law and policing towards them).

Another is the aggressiveness, militarization, and tendency to escalate in the police, much of it brought about by the war on drugs. No knock raids and a propensity to look at every encounter between law enforcement and a civilian, especially a black civilian, as a hostile situation always on the tipping point of violence, and the training to match, has created an epidemic of police encounters that are far more violent than they ever should have been. Police go through training to dominate, to shoot at a twitch if they feel the situation warrants it, and we see the bloody results.

But there's something even darker. And that's the increasing willingness of police forces to commit summary executions on the street of people they feel are, or should be, outside of the protections of the law. Many police have very little qualms about deploying lethal force against "the bad guys", and the public too has been quick to defend the use of lethal force against "bad guys". Rapists, murderers, violent thieves, "scum". And the line on what sort of behavior is allowed has shifted significantly. Since the death penalty was reinstated in 1976 there have been a total of around 1400 legally sanctioned executions in the entire US, for an average of about 35 a year. Police fatally shot nearly a thousand people in 2015 alone. An execution rate nearly 30 times the legally sanctioned rate, and increasing since a low in around the year 2000. This while the rates of both violent crime and of shootings of police officers have been falling. Today the rate of fatal shootings by police is as high as it was during the height of the crack epidemic in the late 1980s and early 1990s, a period when violent crime and street murder (by civilians) were at record highs, several times higher than they are today.

Unquestionably the police have shifted toward a stance of committing summary executions during arrests. Sometimes this happens by accident, where the aggressiveness of the police put them closer to suspects and more likely to be in danger, and thus more likely to provoke a situation requiring deadly use of force. The problem here is that for too long we as a society have been willing to look away (as we were willing to look away when our nation began to adopt torture) from these horrors because we thought that only "bad guys" were being affected. But as we've seen, these patterns of behavior easily bleed over and pretty quickly you start seeing routine traffic stops pushed up against the knife edge of life or death circumstance for no good reason, and good, honest citizens are dying as a result.

This last one is very hard to solve, because, as we've seen, it requires a very stunning reversal of perspective for those in law enforcement. It's difficult to come to terms with the concept that you might be a bad guy yourself, that your actions and your training may be horrifically dangerous to civilized life. So people will naturally shy away from questioning their own training, motives, and actions.