Forgive us, lord, for we know not what we do:
If we see a white guy in (a rich area), for example, and it's 1- o'clock, we just assume that he lives there. If we see a black guy, we're going to stop him because we assume he's checking out the neighbourhood to commit a crime.
So from a policing standpoint, it's sad, but you're trying to look at the community and say, "This person doesn't belong." Internally, we know it's wrong but then as police, we're saying "What if he was about to commit a crime, and we just passed him?"
Which might explain why "do unto others as you'd have them do unto you" is such a universal constant.
Not that we believe in sociology; after all, to assume that there is something greater than ourselves that remains beyond our reckoning - why, that would be heretical, wouldn't it?