WTC 1 & 2 had both internal steel columns and an external steel support structure. If the building was actually hit by a 757 in all likelihood, the plane would have actually bounced off the side.
To demonstrate this, take an empty soda can and throw it at a chain link fence. How long before the chain link fence gives?
To further explain why this would happen, each floor is a poured, light weight concrete sheet. Concrete has amazing compressive strength. And this large sheet supports the outer steel structure from collapsing inward.
So, if the planes (a great big aluminum can) actually impacted the buildings, then the only way in, is to shear a hole through the exterior steel plates.
The fuselage might have made it through in pieces, but the wings, because of their large area would have not. So, if we actually saw a 757 impact the building, we would have seen wings fall after being ripped off.
More than likely, we would have seen the entire plane fall, leaving only a dent in the side of the building.
And to explain to people the center steel columns.
Let us get a 1 ton pickup truck. And a ninja/samurai that can slice through steel.
So, we get the ninja to slice out 1 foot of a support column. And then we have a crane lift this piece, and gently place it into the pickup. The piece of support column fills up the truck bed, and is all that the truck can safely support.
And there was a grid of such support columns at the center of this building.
Just one of these support columns would completely crush a 757, fuel and all, and wouldn't even be dented.