It's as simple as this: Republicans fetishize the police (like they do the military) and will always give them the benefit of the doubt. They have this gauzy teary-eyed love of the police. Just watch Megyn Kelly on Fox to get the idea. Democrats are allied with public unions and will not under any circumstances take on the powerful police unions who fight any attempt at accountability tooth and nail, a behavior Democrats have become habituated to enabling for other unions like the teachers unions.
The issue is mostly about giving police accountability that matches the special powers over the use of force we give them. But it is also about racism. It just burns me up to have folks in power point to the business world constantly for supposed institutional racism, when in fact I witness very little if any day to day. The one institution I see that clearly has elements of institutional racism are many police forces, but no one will touch them.
Every year there are hundreds of police shootings and the number that are determined not to be justifiable rounds to zero. What are the odds there is a process involving humans with this small of a Type I error rate? We are learning form cell phone cameras that the stories we used to believe from police officers about events are often total bullsh*t. And yet still police are not held accountable even when there is horrific video evidence showing them out of control.
At the drop of a hat, at the smallest hint of a single example of a bad outcome, the government will not hesitate to impose enormous new restrictions on private individuals. But even with the most overwhelming evidence the government will not put even the lightest restrictions in itself or its employees.
I have always shied away from my fellow libertarians on the anarcho-capitalist end of things who wanted to privatize the police force. I always thought use of force to be a unique privilege and one dangerous to hand out to private groups. But I am starting to see that I was thinking about it wrong. It is a dangerous power to give to anyone, but at least if you give it to a private party someone might possibly exercise a little accountability over them.
Walter Olson has a good roundup of police and lethal force here.
Postscript: Here is an example of what I mean: The Obama Administration has imposed significant rules on universities to bring greater accountability to sexual assailants when it was perceived that the universities did not impose enough accountability on such predators. I think the Administration has gone overboard in stripping away the accused due process protections and handing justice to people who will not manage the process well, but its the seriousness of this effort I want to point out. While I don't think the Administration's actions were appropriate to colleges, they would represent an entirely appropriate response to police violence. Someone needs to step in and enforce some accountability.