The regulations around radios exist because the spectrum is limited and emissions propagate over a wide area. You and I (assuming you're in the US) have the same standing to use radio spectrum. If I go and modify my phone's firmware to increase the power output I could literally jam communications from your phone.

It's very different than if I modified the firmware on my hard drive or UEFI on a PC. I might fuck up my stuff but it doesn't affect you. I can fiddle with my hard drive firmware all day but I'm not going to block a 911 call you're trying to make.

Also a company giving out modem firmware is an exception and not a rule. It re-classes the device as a hobbyist/experimental device and if they go traipsing around with it they could potentially face fines (unlikely but possible).

Again it's not about lobbying it's about a limited spectrum and people being stupid/assholes not realizing or caring their pocket radio affects others. You live in a world where shitheads try to make their cars louder on purpose and you can pick up dozens of WAPs because everyone sets the power to the highest number the interface allows.

Every radio regulation agency on the planet? Most radio hardware is capable of operating outside of regulated limits. The device firmware is usually what keeps the devices running within their regulated limits and gets those components licenses to be sold. Anyone selling regulated devices running outside of their regulated envelope faces fines and even criminal charges.

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.