Look obviously the baseband is under control of carriers. That's required since they manage spectrum, you know AT&T's "one phone could disrupt service for an entire neighborhood" argument. Which is true, btw.

This includes the power to upload code to decide which channels and timing to use.

Then it was decided to use this for law enforcement, and so audio was routed through the baseband. Other things were for carriers, like SMS management (including deleting SMS that were already shown to the user). Both to prevent apps from listening without the baseband's agreement AND to listen in without agreement from the apps.

The limit on this is that there's already many different basebands, and of course neither carriers nor states could be bothered to actually implement the backend necessary. I'd bet good money the NSA has one though.