The prison system sucks in so many ways. Legal slavery with archaic rules that would be considered hate crimes or human rights violations if they happened to people in the U.S. who aren't incarcerated.
As far as old cars with cassette players. Like you said, you can use an FM transmitter, but I also remember having a fake cassette with an aux cable so you could plug it into a CD player headphone jack. I would bet they have a bluetooth version these days, so you don't have to listen to cassettes even in those cars.
That reminds me of something though. When I was a kid, we had a Toyota Corona station wagon with an 8-track player. My father had this converter device that you plugged into the 8-track and it had a little tray that you laid a cassette into so you could play it. I don't remember if the sound quality was worse than playing a cassette in a player designed for it.