How did the plane that hit the pentagon get that close to the ground?

Often when planes fly into Ronald Reagan Airport in DC, they will approach from the northwest and fly a route which follows the Potomac River. If you're ever on board one of these flight, you can see that there is a nice view (out the right side of the plane) of the pentagon from this flight path. After clearing the large buildings of Arlington, VA, there is/was a clear shot for the plane to get to the pentagon without crashing into any other buildings.