Terrorism? Please. That's the most overused excuse in the book.

The perimeter rule is actually a legacy rule back from before the days of the A319/A320 or Next Generation 737s. The goal of the rule was two-fold: 1) to facilitate the growth of Dulles since it was in the middle of nowhere, and 2) to ensure smaller aircraft at DCA due to noise concerns as the surrounding area is quite populated.

The Dulles issue is no longer an issue, as that area has boomed, and it's actually busier than DCA now, IIRC. However, back in the 70's, Dulles was a joke, and nobody could believe anyone would build an airport out there.

As for the aircraft noise concern, the current rule is actually counter-productive to what they were trying to do. In the past, to fly pretty much anything over 1,250 miles, you'd need a 757 or 767. Now, since most newer aircraft have the range for a transcon, and are quieter than older aircraft, by requiring short flights, you are more likely to have an older, noisy aircraft serving DCA, because airlines will use their newer aircraft for longer routes.