I'm going to guess the concept of rule-of-law will escape them, so try this:

They're the Transportation SA, and have already started doing trains, subways, and buses. "Random" searches of individual cars on interstate highways (or residential streets) is not out of the scope of their mission (as defined by the TSA).

If they can't grasp scope (any place, any time, any where, any mode of transporation), try scale (your car is next).