I think the counter argument is that the carriers must be automatically cutting back their use of the spectrum when they detect other devices, so if unlicensed

I think the counter argument is that the carriers must be automatically cutting back their use of the spectrum when they detect other devices, so if unlicensed devices were allowed on the carrier licensed bands, it would reduce spectrum available to the carriers. Though it seems fair, if they want to eat into the public unlicensed spectrum, they ought to give the public access to their own spectrum.