There used to be a limit on how far you could fly from Reagan National, Dallas was beyond the limit.
So American had a flight that stopped at Dulles for just a few minutes, a few passengers got off, none got on. I would be booked to get off at Dulles, but if there was a happy hour downtown, I would stay on for the trip to DCA. The flight from IAD to DCA was only a few minutes, the price was the same.
It was legal either way, American didn't care.