If (a) there is no definite speed limit, just an imprecise policy statement (like "don't drive too fast") and (b) a person drives by the same cops for years at the same speed and gets clocked and doesn't get ticketed, and then (c) is one day pulled over and told that they're driving imprudently, then they indeed have a basis for complaint. That is exactly how this works. The interplay between vague policies and regular practice is how people gain clarity in the absence of a bright line rule -- playing ball.