Sure its the rule, and that we have to abide by them, and of course we do. The question of the OP though is not whether you should or do do it, its whether you would go as far as to report them and/or tell them to turn it off. Airlines also have a rule about having your seat upright during takeoff and landing. If my neighbor's seat is 1 inch back, am I going to call over the F/A to get him off the plane.
Plus if there was a policing force (sure we're entering the realm of awesome sci-fi movies of guys with dB signal detectors), wouldn't it be FCC enforcement agents, since its the FCC that controls that sort of thing....