500 pound payload doesn't include pilots and fuel. Two pilots and full fuel and you're overloaded. These little training planes are constantly running light as possible to stay within weight and balance limits.
Fine, how were our security people suppose to confirm that was the case? Stip out extras to add capability. Did the have a full tank, or only enough to reach DC - one way trip? Or may be even the much talked about 'suitcase nuke' Too many what ifs to ignore.Add to that the symbolism of a strike even w/o explosives.