Yes, some networks can exist but not in the exclusion zones.
There is a very good reason to not support them:
1) using band 12 devices but only transmitting on band 17 - what ATT would have to do - would degrade the signal in the exclusion zone.
They posted signal strength/filter charts showing that Ch 51 is too strong for band 12 filter; they need band 17 filter.
2) mandating dual band 12/17 on all such devices would take up a port on the transceiver/modem/something like that.
(The following statement is a guess)
This would leave ATT one less port to use for Band 29 (D,E block) supplemental downlink.
(End potentially clueless statement).
I think the FCC should have not sold the A block at all until Ch 51 was gone. That way, at least all those A-purchasers would still have their $$$ which they could use for refarming current spectrum.
Furthermore, the A-block purchasers were clueless and lazy. They should've known about the exclusion zones, they should've known about the bleed-through into B,C blocks and they should've done engineering tests around channel 51 with band 12 devices (or done simulations).
Unless AJ wants to chime in, I'll assume this viewpoint is correct.