for whatever unknown reason you can only use it on AS metal with one exception and Cancun isnt it. Possibly it screws up something with the other carrier either with the reservation or the money. Or AS just wants to at least get your money from the higher fares they typically charge.
Likely because AS would have to pay the other carrier for two seats, yet they'd only collect revenue for one. When it costs AS some "funny money" in the form of a possibly-otherwise-unoccupied seat, they're happy to let you use it, but when it costs AS real cold, hard cash paid out to another carrier, they ain't so happy.
