Even if the structures of the towers may not have failed due to the impact of the aircraft, they were both transcontinental flights. That means that even with the takeoff and travel to New York City, they still would have had a nearly full load of jet fuel. The fire caused by the fuel could have weakened the structure of the building to the point that the structure could not hold up the several floors above the point of impact.

As for Tower 7, there was structural damage caused by the collapse of the towers and fires burned on the inside. Inadequate fire suppresion systems caused the fires to burn out of control, weakening the structure of the building. If almost ANY building were allowed to burn uncontrolled, the frame would eventually weaken to the point of giving way, and in this case, it brought down the building.