" "Why would rappers want to sully their art by associating it with folk music? " "

Do you honestly think anyone cares? Unlike folkies, rappers are not spending time on forums trying to figure out what they are.

Likewise, I doubt if a tomato worries if it is considered a fruit or a vegetable.


" Have the cops tell them "you are free to listen to that stuff anytime you want...just wear earphones". "
You may wish to check your local laws. The last time I heard, it was illegal to wear headphones when driving.

"no need to share the "Cool" with others that don't like it don't need to hear it "
As long as the levels are not out of spec with local laws, there is a freedom of speech issue involved with telling someone they cannot play music in their own car.   

Comeon, haven't you ever turned the volume up to 11 on certain recordings for the pure joy the sound brings? Ever try to listen to the Rolling Stones or Ramones on "soft"?   Doesn't cut it. I cringe when I see two guys with banjos playing together and I run in fear when a tone deaf geezer sticks his finger in his ear and sings with all the gusto of a cat being castrated, but I'm not going to make a fuss. Move on, life is too short!!!