This "rule" is a pathetic attempt to address a very complex situation with a very simplistic answer. Anyone who has flown out of DCA knows that by the time they finish climbout you are at least as far away as planes leaving IAD. Why these silly restrictions only at DCA? Most DCA departures I have been on climb steeply away from the city. Unless an attacker was already in the cockpit they would find it very difficult to take control of a plane for at least several minutes after takeoff. I do not know how fast a typical comm'l jet moves after takeoff but I'm guessing it has to be at least 3+ miles per minute. By the time an attacker works his way into the cockpit you would likely be at a minimum of 12 to 15 miles downrange.
To those who advocate closing DCA - what about all of the jobs lost, the 500+ million US spent in renovations over the last few years, the overcrowding at BWI & IAD once air travel picks back up - and it will pick back up - it will just take time. There are only 600 to 700 members of Congress but millions of regular pax that use this airport. Closing it would be a huge mistake and be giving in to terror.