I wonder what the logic was when the system was designed? Why not let the rear seater be able to tune the radio and change the volume? Doesn't make much sense to me, oh well. Thanks for the responses guys. Much appreciated.
It may have to do with a little 'safety' consideration toward the operator. If I'm not in the mood for high volume, and want to concentrate on the road noises, my passenger will just have to suck it up. Or, they are welcome to plug in their own headphones and source of music, so that I don't have to hear it. But, only if they can still hear me if I pass back instructions over my shoulder without me having to repeat several times.
The operator of ANY motor vehicle should be the only one who controls the operation of said vehicle... passengers may make suggestions, but it's ultimately the operator that needs to focus on the responsibility that their passengers' safety is their responsibility.