Boom Box - Why use a speaker when (if) you can't get the most out of it?
Passenger - Can you turn up the controls on that radio?
 
 

Okay.  I have noticed this, as well.  My theory is that it sounds rich to the point you can feel the bass.  In a new/rental that's fine.  But, if I'm buying the car it is entirely possible that I would not want my speakers blown the first time I turn up a favorite song!  And, of course, we all have our own taste for bass, etc....
 
Maybe someone, like the detailers, who likes rap or heavy duty electronica might prefer to keep the settings extra amped...however, most listeners would turn those settings down for more mainstream music/applications.
 
My Guess....for what its Worth.

Whenever I rent a car, I usually plan a surprise for the kids at the rental counter. I scan for that one really odd station. You know, the one that plays polkas, barber shop quartets, Siberian Deth Metal, and Industrial Folk music? I tune all presets to that one station, turn the treble up all the way and cut the bass and mids. I make sure to leave the radio on and cranked all the way up when I turn off the engine. 
 
Thinking of the car jockey's reaction when he goes to move the car makes me smile. That's about the only use I have for a car radio.