It may be "obvious" that a heavy plane hitting a skyscraper would deliver a "tremendous" shock, but it doesn't follow that the building must therefore collapse. In 1945 the Empire State Building was hit by a B-25 bomber, but it was still standing last time I saw it. "Ah yes, but it was the impact plus the fires!" Well, when the B-25 hit the Empire State Building "its fuel tanks were reported to have exploded, engulfing the 79th floor in flames",
These two situations aren't entirely comparable.
The maximum weight of a B-25 ranged from 27,100 lb to a limit of 41,800 lb, for instance (see www.warbirdsresourcegroup.org/URG/b25mitchell.html). A 767-200 ranges from 179,080 lbs (empty) to 395,000 lb (maximum takeoff load) (www.aerospaceweb.org/aircraft/jetliner/b767), and FEMA said the 9/11 planes had “an estimated gross weight of 274000 pounds” ( http://www.fema.gov/pdf/library/fema403_ch1.pdf ).
The maximum speed of a B-25 ranged from 275 mph to 315 mph, depending on which version it was, and as the B-25 pilot was trying to avoid the building it's unlikely he'd have reached that (cruising speed was 230 mph). On 9/11, "American Airlines Flight 11 crashes at a speed of roughly 470 mph" and "United Airlines Flight 175 crashes at a speed of about 590 mph" (http://channel.nationalgeographic.com/channel/inside911/timeline.html), a considerable difference when you factor in the extra mass as well.
The B-25 had a "normal total fuel load of 974 US gallons" (http://www.b25.net); a proportion of this would have been used already in the plane’s flight. By comparison, "it has been estimated that both UA Flight 175 and AA Flight 11 were carrying about 10,000 gallons of fuel when they impacted" (www.serendipity.li/wot/wtc_demolition_init.htm).
The end result of this was considerably less impact damage, as this photo shows.
Less structural damage meant no real issues in terms of supporting the load of the building above (which was constructed entirely differently from the WTC anyway). The considerably reduced fuel load meant fire was less of an issue, and the blaze that did arise was brought under control without much difficulty:
The 4-alarm fire brought every available piece of fire-fighting apparatus to the scene. As the building was evacuated, firemen spent about an hour extinguishing the flames.
At first glance it might look like the B-25 crash has some relevance to 9/11, then, but the facts say otherwise. The two events bear very little comparison, and it should be no surprise that they also had such very different outcomes.