Dont know why do they do that, but I would rather have two ZED tickets, one in C and other in Y. when they clear you, ask the agent if it is a seat in economy class (if it is not shown at the boarding pass), and give them the ZED in the correct class.
I suppose that happen because of the domestic free upgrades for elite pax... dont really know if USAIR does it, but delta does, and it is hard to get a seat in J or F when flying domestic.