Why do you think police men abuse their power and what should be done about it?

I recently saw these two You Tube videos of police men losing it on skateboarders literally just for skateboarding:
http://www.youtube.com/watch?v=aFUpa0OwlyU
http://www.youtube.com/watch?v=1hxOr3q7nrk&feature=grec_index
In one video, they were just skating and minding their own business. Suddenly they were yelled at, and one of them was choked and arrested. When one of the girls tried to talk to the police man about it, she was grabbed in a choke hold and arrested as well, as was another friend of hers for trying to protect her from the police man’s choke hold. I saw another video where a police man lost it on a teenager for calling him “dude” and “man”
I have heard many other stories about police men abusing ther power and not listening to kids, why do you think this happens? Are there police men really just jerks or is there something that happens in training that makes them act that way?
In most cases, the police man only gets suspended for a couple days with pay. I personally think that is just wrong.
I don’t know what to think about this because ever since I was a child I put my faith in police men to keep me safe, but now i am afraid one day I will be harassed by one for no reason. I have been known to have authority problems, so I don’t like police men to start with. I think any police man that abuses their power needs to be suspended for at least a week or two with no pay or lose their job, depending on how bad things got.
Why do you think this happens? What are your opinions about it?