That's really weird.

When I flew home in June on my dirt cheap BUF-BWI-BUF ticket, I decided to show up at DCA and try to grab the flight out of there instead. (It was more convenient for me as I was staying in Crystal City). The agent at the DCA ticket counter first called the rate desk - who confirmed that my ticket permitted travel only to/from BWI. Then the agent went to speak to a supervisor in the back, who approved my request to fly out of DCA. Maybe they bent the rules because I'm a Gold member, who knows? It also helped knowing that the DCA-BUF flight I wanted was only about 30% full.