Here is something about police unions many, many people do not understand - unless specifically mandated by by-laws or rules of the union, they have a legal and fiduciary responsibility to their members, and unless these by-laws and rules allow, many times they must represent officer's and their misconduct or else they could be sued for a failure to represent. 

Strangely, for the longest time, our union had a by-law that allowed the union to avoid providing an officer with legal representation if the misconduct or behavior was ruled non-job-related by our union's executive board.  We then had an uprising within our membership over complex issues and our membership then chose to change legal representation when the new legal representation persuaded them they could offer more if the change was made.

So when these politicians, activists (and even police management personnel) go on about unions protecting bad apples, it is mostly because of the fiduciary and legal requirements found in paying union dues.