The DHS says that it's against the law to photograph "sensitive" government buildings, but they won't publish a list of these buildings, so it's impossible to comply with the law. The rub is that if you get caught breaking the law, you'll get shaken down, have your name and personal information taken, and go into a file, presumably forever.
The bottom line is that McCammon was caught in a classic logical trap. If he had only known the building was off-limits to photographers, he would have avoided it. But he was not allowed to know that fact. "Reasonable, law-abiding people tend to avoid these types of things when it can be helped," McCammon wrote. "Thus, my request for a list of locations within Arlington County that are unmarked, but at which photography is either prohibited or discouraged according to some (public or private) policy. Of course, such a list does not exist. Catch-22."