I'm sure some of it is national security BS. But that would generally only apply to the governmental core of the city, not the rest of the residential and commercial parts. I think that unlike some other major cities (New York, San Francisco, even Baltimore!), DC either lacks a Motion Picture Commission or they don't have their act together. I don't remember the name of the film, but oh perhaps 15-20 years ago there was a movie that took place in DC and had a chase scene on the DC Metro. Metro wouldn't let them film in the subway so they had to use the Baltimore subway as a stand-in. I think the excuse at the time was that Metro didn't want to depict crime happening on the system (at that point in time it was in fact one of the cleanest, safest systems anywhere). Since then, they've changed their mind, and in State of Play (mentioned above) they actually do have several scenes that take place on Metro and actually show the DC Metro system (although they filmed in stations that were not the ones alleged to be in use - that's a practical thing, as the one they used is a much bigger station with a bigger platform that made it easier to film).