I assume because of the baseband stuff to be FCC compliant? Last I checked that meant DMA channels, etc. to access the real phone processor. All easily activated over the air.
FCC devices are certified / allowed to use a spectrum, but you must maintain compliance. If you're a mobile phone manufacturer you have to be certain that if a bug occurs, the devices don't start becoming wifi jammers or anything like that.
This means you need to be able to push firmware updates over the air (OTA). These must be signed to avoid just anyone to push out such an OTA.
The government has a history of compelling companies to push out signed updates.
Indeed. The only reason this is not used by customer support for more casual access, firmware upgrades and debugging is a matter of policy and the risk of mass bricking phones and as such this is not exposed to them. There are other access avenues as well including JTAG debugging over USB and Bluetooth.
If there's one thing we learned from Snowden is that the NSA can't break PGP, so these people who live in the world of theory have no credibility with me.
I never saw a single speck of anything I ever sent to anyone via PGP in there. They had access to my SIGAINT e-mail and my BitMessage unlocked, but I used PGP for everything on top of that.
Stay safe!