I've finally done the research into where these settings come from. They're part of the factory image (or OTA, presumably). USMobile doesn't even have its own dedicated settings - it looks like we're just getting the default Verizon settings. The ability to select network type was removed for all Verizon customers in the June update. I checked both the generic and the Verizon carrier versions (I hoped it was a mistake and was fixed in the carrier version).
This is a very bad finding. It's just Verizon being Verizon. I'd be pretty surprised to see USMobile get preferential treatment.
The lesson as usual is: you don't own your phone if you don't root it.
(Technically speaking, unlocking the bootloader is enough.)