Why is it that those on the left who believe that the police and the military are not held accountable (e.g. Ferguson, Treyvon Martin) constantly act is if they believe every other branch of the government acts with pristine purity in every aspect of their jobs? If that were not the case they would not constantly clamor for more government involvement (both in terms of money and regulation) in every aspect of government?
More money for education. More money for climate change research. More regulation for health care. The list of bigger government requirements goes on and on.
The right has in the past been somewhat guilty of this as well, but in reverse. The military and police can do no wrong, but every other part of government is flawed and requires downsizing as a cure. But the right has made strides in this area - the military is still important, but it is equally important that they be efficient and not waste taxpayers' money. The police are important but they cannot be intrusive of personal liberty.
The right has evolved, the left has not. In fact they've regressed. When it comes to the police in particular, there's all too often a presumption of guilt until proven innocence, and even then, they often still presume guilt.
There's no simple solution to this as it involves perception and world-view type stuff. But it is something that needs to be addressed.