The radio portion cannot stop you from using tethering though. Also, as far as I am aware, radio firmware on Android is often stored on an internal MMC card.

the radio portion is quite different as there are externalities tied to it. the phone portion doesn't

Is it not the very exact externality (interfacing with the carrier's network) that is being discussed here?

Not exactly. Radio chip firmware is responsible for controlling the physical layer, and restricting access to it is a practical compromise that ensures people don't generally cause RF interference for one another.

The problem discussed upthread is on the application layer - you not being able to run arbitrary code on your own device because companies prefer to please one another rather than their customers.