Originally Posted by seawolf
That means you were probably travelling on two one-way fares combined onto one ticket. The return portion of pure round-trip fare can not be used on another itinerary involving a different city pair market. To allow so (even with change fee penalties) would cause a breakdown in the pricing difference between OW and RT the airlines have in place.
Seems like on many fares, UA (and other majors) now have simplified their fares so that quite often r/t's are indeed built from combined one-way fares. I know this to be the case on any route they're competing with a LCC on, such as just about all of the UX destinations out of IAD... check anywhere DH flies vs. where they don't o/w, and you get an amazing difference in fare.