It's physics. Buildings are not blocks of steel. They are buildings. If the upper portion of the building is suddenly allowed to go into freefall because an airplane hit it and cut the structure supports so that the upper portion came crashing down on the lower portion (NIST's claim) then the lower portion would push back, according to Newton's laws of motion, and the lower portion of the building would destroy the upper portion of the building as the upper portion was destroying the lower portion and it would not crash to the ground or pick up speed or accelerate. It is physics. It is 7th grade physics.