You've basically repeated what I said only in a more PC way: he cannot trust anyone's side of the story verbatum. As you say, he has to listen to all and then make his decision, sometimes in less than .001 seconds.
If you ever watch the "swat" series on TV (Dallas swat, Detroit Swat etc) you will see how the cops act: they go into any situation and immediately control it. If they trusted everyone then they would have no need to control any situation and could just let the perp(s) freely admit that they are the perps, what they did, and put their hands behind their back ready to be cuffed. Of course this doesn't happen so they get control of everyone until they determine what is going on.
Have you ever noticed that even on the show "cops" that if the person acts decently, many times they actually are let go? Often with just a warning, a ticket or a citation?
I keep saying this over and over again, the second you resist a cop from controlling a situation in any shape or form you open YOURSELF up to a world of hurt.
For the encounters here of cops not performing their duties in a satisfactory way, let me ask you: how many times have you had a cop perform perfectly towards you? If those were the only times you've encountered the police then hey, your opinion is skewed.
Here's a concept for you: anytime you think "fucking cops, they're all crooked and assholes" drive by St Jamestown, Jane Finch or any of the other high crime areas. Just drive through it. The ONLY reason you can do that and not die is because of all those rotten assholes doing their job. Cuz if they didn't? You wouldn't make it 10 feet........