Now a rational society would tie those kind of things together.

That would have been a good reason to refuse to allow the planes to get that big. And since buildings like that are built with a view to staying up for a long time, it's necessary to take into account future technology, such as bigger planes, when building them, and allowing them to be built.

In fact the Twin Towers design was finally approved in 1966, when construction started; the first Boeing 757 was ordered in 1978. That's not a long time in the life of a building - I am sure that any aeronautic expert in 1966 would have been able to predict that, within the lifetime of the Twin Towers, planes as big as that and bigger could be anticipated. That would imply that if a building like that was to be built, it would have to be able to withstand that kind of collision. (And I'm not suggesting that they would need to have been thinking in terms of terrorism, but rather of accidents, which do happen).