I'm in the philly area, I know the incident, and I followed that one through the news. edit: and again: the union wasn't who aquitted him, was it?
But again- the union is not the cause of the problem here. Here's an article that illustrates a lot of what is:
http://www.slate.com/articles/news_and_politics/politics/2015/05/the_myth_of_the_hero_cop_police_unions_have_spread_a_dangerous_message_about.html
Slate has issues, but they're on point here. There is a culture of deference towards cops in courtrooms, the legislature, and the public that gives them the benefit of the doubt when they fuck up.
Unions didn't pass laws that protect cops (like the 72 hours to get your story straight before you can be questioned business) your legislature did. Unions don't sit on juries, your citizens do.
All the union does is fight for due process when things happen under the terms of their contract, and the contract takes a backseat to the law.
edit: and all of this assumes that "private cops" that aren't union don't exist. They do. HOA's employ them quite a bit to keep "undesirables" out. They don't behave any better than public officers do, and there's less accountability.