Serious question, I'm honestly not trying to insult anyone in particular on the board, but I think this is a fair question from another side of the argument.
Here's my outlook, so you can determine my 'bias' right from the start. I think that there is probably the same percentage of a-hole cops as there are a-holes in any profession (doctors, mechanics, accountants, pig farmers, etc...). MAYBE a bit higher percentage due to the high numbers of males in the profession and the nature of their work combined with the day-to-day shit that they see and have to put up with. Thus an accountant may be less likely to turn into a jerk, because he'll never go to work to see the body of a child who has just been killed by his father, or have a perp pull a gun on him during a traffic stop. Doesn't make it right, just puts the behavior into a new perspective.
Now when I was a teenager, I thought the world was against me, and most cops seemed like a-holes to me, even though I NEVER had any experience that I would consider a violation of my civil rights. Sure I thought I was 'harassed' once or twice, but I probably deserved it for being in the wrong place at the wrong time with the wrong people and having the wrong attitude. Even then, it was never more than a 'move along' type of response from the cops who were 'harassing' me.
As an adult, I have never had a 'bad' experience with a police officer. Sure getting a ticket is a 'bad' experience on my end, but the officer was always respectful, and I showed him the same respect. Plus, I did the crime, so I should face the consequences, right? Hell, I've even been let off by officers with a warning several times, so why should I complain?
So why is it that some people always run into 'bad' cops?
My theory is that 'a-hole' people will more often be doing the activities that attract the most attention from cops, so after a while, they will begin to view any encounter with the cops as a negative one. If you compare two people of similar race and background, and present them to the same police officer in the same situation. Person A (clean record) will describe the experience as neutral or positive, while B ('a-hole' with a record) will describe the same experience as negative. Even if the cop is being polite and respectful, it is more likely that person B will do his best to escalate the incident and generally making the experience longer and more painful for both parties.
I base a lot of this theory on the experiences of an a-hole I had the displeasure of knowing. The guy had about 4 DUIs (usually accompanied by possession charges) charges for resisting arrest/assaulting a police officer. It always baffled me that he hated cops so much because he based his hatred on this one incident: he was piss drunk, wrapped his car around a tree, and when the cops responded, he grabbed one officer by the throat and slammed him against the police car (the guy I knew was an ex-college football player, 6'+ and just under 300lbs). Although he should have been happy that the cop hadn't shot him on the spot, he forever hated cops because the second cop OC'd him when he had the first cop by the throat! He said they were 'pussies' for spraying him, since there were two of them and one of him!
But I am of course assuming that not all cops are a-holes, if you don't agree with me, then that blows my theory out of the water...