Like all humans, their view can become biased by what they hear first, but all the cops I know and have known cared about the truth. Like all of us, they can be mistaken, but that's different from being deliberately biased.Ok, lets get one thing clear. The police DO ( in the UK and I've no reason to doubt it's any different in the US) actually care about the truth. They will work long hours and go out of their way to make sure the correct person is charged for the crime. They aren't helped by numpties who hate the police and constantly run them down because they've had 'dealings' with them in their dim past and feel slighted ( mostly because they were caught doing something they shouldn't)
Note I said charged not prosecuted because in the UK the Crown Prosecution Service is responsible for prosecutions not the police. While there are mistakes made by the police ( show me anyone who never makes a mistake and I'll show you a liar) the police do their damndest to do a very difficult job to the best of their ability. Yes there are bad apples but they usually do get found out and busted.
Slagging the police off is a pointless and stupid thing to do...unless of course you can do the job better especially in the light of the shortage of manpower and the budget cuts.