Not all of them are actively abusing citizens or running protection, but the thin blue line means even the "honest" ones tolerate it.
If I were to look the other way while my coworkers abused or stole from people, I would at the very least expect to be fired and not trusted with similar responsibility again. That is what a normal citizen looks like, and I don't think a normal citizen attitude could survive in a cop shop.
There is the additional problem that your criminal cops (and we both know there are plenty of them) don't exactly wear identifying tags, so I have no way of knowing which are which. That means my only strategy is to minimize contact with and distrust all of them.
This is simple: if you want people to respect and work with the police, stop protecting the bad ones.