Similar to having a device that lets you listen to police chatter to see if any are nearby - ooo wait that would be illegal. Anyway i can see your point, 1 man doing his best not to get caught is fine, a man helping someone else to not get caught is wrong. In the ever famous brick throwing situation its more like a man see's the police on the look out for trouble down a street, a line of men walk past with bricks in there hands. He warns them all of the police just incase they are not building a wall but have other plans by fluttering his eyelids. The police them nab him for warning these potential brick throwers of them. So the police don't actually need to prove that anyone was actually going to commit a crime, just more of a 'just in case they', a preemptive strike so to speak.