My car does this and it's annoying as crap. Luckily it is easily disabled. Audi calls it GALA - "Graduated Audio Level Adjustment."

In my VW it's linked to the car speed and I would not want to give this away. Turning the volume up and down when entering the highway or a slow stretch was really annoying on my old car.

I would think the best version would just be a button you push when you want to adjust the volume to a “reasonable” level. I agree that continuously varying the volume would be annoying.