I think there's some truth to the radio thing, and it transcends commercials, shock jocks, Rush, and everybody in between.
Radio, much moreso than TV, forces you to actively listen if you're going to actually understand what's being said. There is no visual guidance, no visual cue or direction. You either hear and process what's going on, or you don't.
With TV, seems to me, you can be much less involved and still call yourself a "watcher."
I really need to get a small radio for my office at work......