LEO do routinely believe that people they investigate must be guilty. They don't investigate at random, but rather are prompted by some evidence of criminal behavior. That certainly doesn't mean that you must be guilty if they investigate you, but it sets them up to believe you are.
The system is set up with compartments so nobody sees the full consequences of anything and this sort of misbehavior is encouraged. If you send somebody to jail, they disappear from your life the moment the verdict is reached and you have no reason to dwell on the harm you've caused them. (And if you do, you probably think it's a good thing, because you think they're a criminal!) And you probably get a commendation for putting a bad guy away.
Bad behavior does get reported disproportionately. Most cops are good cops. For everyone like this, there are ten cops who find a lost cellphone and personally deliver it back to the owner, as happened to a neighbor of mine a few months ago. That doesn't make the news. Although that doesn't mean it shouldn't be a concern for the majority of us. Even a small risk of being falsely imprisoned is important, and there's no harm in taking the "SHUT UP" route with a good cop.
Finally, I would be wary of assuming that malicious intent is no higher than among the general population. Bad people tend to seek power over others, and what better way to wield such power than to do so legally as a member of the police? No doubt, the legitimate "protect and serve" folks still greatly outnumber the bad guys, but I would bet they're more concentrated.