They cannot without materially damaging their current contract partner AT&T. If they were to have a future contract with Verizon or any other carrier, and were to advertise that fact, AT&T would have a restraint of trade case against Apple that would be a slam-dunk in the courts, as they could easily demonstrate that customers who would consider switching to AT&T would decide to wait for the announced iPhone on (insert name of carrier her). Instant proof of financial damage... and fodder for a lawsuit.
Until the current contract term is complete, Apple must abide by that contract and the partnership it entails. Unless AT&T releases them from that contract, or materially violates the terms of that contract, Apple is stuck with them.
As always, thank you for your clarity.