A police officer is not qualified to determine who is guilty. Only to determine who is suspect. All suspects are to be treated alike. And of course, we can all come up with blatantly obvious cases where someone is guilty of some heinous crime. The point is that there are many more cases where the crime is less noteworthy, the guilt is less obvious and someone still gets treated badly before his guilt has been established.
And many times this means they think you're guilty of things that shouldn't be a crime in the first place, like possessing drugs.