It isn't a scam at all. There are three airports which serve the Washington area.

DCA - For the British, this is like LCY. It is small and the only international flights are on small jets to Canada. No transatlantic service from this airport.

IAD - This is Washington's main airport, although like Heathrow it is well out of the city. It is about 26 miles and 39 minutes from Washington.

BWI - This is Baltimore's main airport, although because Baltimore is so close to Washington, it is still relatively close. It is 30 miles, and 46 minutes away from Washington.

BWI was recommended by someone because it isn't Washington's main airport, so it doesn't receive a lot of international flights and is therefore more efficient to manage the inside-the-airport experience.