Looks like they may have you here. That darn old contract of carriage. I'm not saying it's fair but it's out there to read and no one ever does.
"Except as stated above, all nonrefundable tickets have no further value once the scheduled date of departure
for any ticketed segment has passed and the customer has failed to travel on such ticketed segment,
other than due to an oversale, a cancellation, or delay of the ticketed flight by the carrier. All unflown segments
of such tickets are invalid and may not be applied toward the purchase of another ticket."
Article 5.1 on page 15.
http://www.usairways.com/pv_obj_cach...f_carriage.pdf