Guilty until proven innocent?
I see people driving cars all day long, how an I know if they have a license? Can I pull over every car and run them?
Unfortunately, in these "modern" and "Oh, so sophisticated and civilized" times, "everyone is a potential enemy until proven differently" is the best approach to long life. LEOs have to assume this even more than your average man on the street. Particularly when general respect for the Law has gone the way of the horse and buggy. (... and why many LEOs, over time, submit to the general "them versus us" mindset when dealing with civvies. I can relate..... having spent the 70's regarded as somewhat like a plague carrier by the average civvie instead of a career military professional.)