The supporting beams of the WTC towers were loaded in the vertical direction when the impacts occurred. Now consider 80,000 lbs travelling at 500 mph slamming into those beams horizontally. The beams sheared off at the point of impact as can be seen by the massive holes that were created. The energy contained in the planes was transferred to the sheared pieces of the beams which is what sent them flying from the point of impact. Take a piece of drywall and stand it vertically in an open space. Now swing a baseball bat at it as hard as you can and see what happens. That might give you some small idea of how the forces at work apply. Since those other buildings you are referring to where not hit by planes you are trying to compare apples and oranges. I am not defending anything. I am applying my own engineering knowledge to what I witnessed with my own eyes that day.