Pairing makes sense for sensitive data usage, such as tethering. But it's a broken pattern for consumer purposes like multimedia and gaming.
Make it so it doesn't interrupt your headphones and then all of a sudden it's trivial for anyone to listen to at least half of your telephone calls.
Most BT headsets already pose a serious everyday security risk to people who might be targets of surveillance by as minor players as private detectives or journalists due to both protocol and chipset weaknesses. BT security has been improving since then, but the last thing it needs to do is go backwards because you can't be bothered to pair a device with your car once(!)
with open radio, anyone on the train could be saving a hi-fi copy of the entire conversation.
in your analogy, he would have to be pushing a recorder between your ear and phone. very different from sitting in your door steps.
but i agree, the current system is crappy. I never bother to use bluetooth. but removing pairing instead of fixing is dumb.