Cell phones only work because the millions of devices run within strict limits and behave reasonably. There's not a lot of difference between a properly operating radio and a radio jammer. Purism isn't going to find a baseband vendor that's going to risk their licenses by allowing for open source firmware.
As far as I know, there is no licensing whatsoever for baseband makers?
Where did you get that it is?
This is why a baseband processor is a fully separate component from a device's application processor(s). Since the AP doesn't talk directly to the radio it doesn't need to be certified and can be updated without recertification. The BP can also get certification and any manufacturer using that BP doesn't need to re-certify it. The interfaces are also such that the AP can't (or shouldn't be able to) tell the BP firmware to boost the output power above legal limits or something.
Radios that have "open" soft modems don't typically have fully software controlled radio front ends. The radio front end will have its statutory limits baked in electrically or have very limited software control. The modulation on the back end isn't as important as the front end. Broken modulation just means you can't talk to anyone, an overdriven transmitter is effectively a radio jammer or can give someone an RF burn.
Can you point where it is stated?
