That scenario makes no sense to me at all. They should have let you onto the DCA flight. This is also news to me, as I am based at FLL and have been rerouted at times to MIA or PBI when a FLL connection goes bad. In addition, I've noticed that the deeply discounted fares into DCA can often be as low as those into BWI, so the argument made isn't necessarily even true.

Not a good example of customer service and the fact of the matter is that U would have saved money by putting you onto the DCA flight assuming that there were open seats. Maybe you got the wrong agent working on this...