Yes, they were right. If you read the fine print in the rules for non-refundable fare basis codes, you'll see this mentioned at least once if not more. I routinely end up with unused nonrefundable tickets as I change my plans often. This happened so often, in fact, that I no longer book more than 6-7 days in advance -- doing so was actually costing more than it was saving, especially after DL implemented its new fare structure system-wide.

Here's a snippet from one published fare:

"IF NO NONREFUNDABLE FARE WAS FOUND THE PASSENGER WOULD HAVE TO PURCHASE A NEW REFUNDABLE TICKET. THE NONREFUNDABLE TICKET MAY BE SAVED AND APPLIED TO A FUTURE PURCHASE OF ANOTHER NONREFUNDABLE TICKET PER G/NWR/GEN/P4."