Now hear me out.
Because i think it should.
Humor is a natural way of dealing with an event without it necessarily being meant to cause offense. If something can be mocked then it removes some of power and fear it holds over people. It removes part of the wound dealt by the event.
In a very literal sense of the word, humor can provide much needed Catharsis. It stops us dwelling and being paralyzed.
9/11 on its own isn't funny. At all. It' tragic and brutal and deserves the up most respect and reverence. But on the other hand the overreaction and use of it as a justification for things as diverse as punitive legislation on civil liberties (see; patriot act), to it being used as a blanked 'go to' button for certain groups to hide behind is very ripe for lampooning. As is the insane ideology that lead to the attacks. If you can make even tenuous reference to 9/11 in a policy or statement it is seen as being unable to be attacked. That's more insulting than a tasteless joke, it invokes the a horrible event not to turn it into dark humor, but to channel the memory of those people into a selfish agenda.
And that's partly why people make 9/11 jokes, or jokes about it's effect, it has been used as a 'trump card' and ultimate scared cow for so long it's taken on almost mystical powers. The only real to deal with the event it is to accept it and try and move on. Humor can be part of that.
Comedy and satire can also make very serious points more palatable. With humor we can call attention to some of the absurdities acts of violence. "Dr. Stangelove" is the prime and probably the best example, that film made nuclear war funny and absurd at the height of the cold war.
Terrorism as a subject matter, even when it is very close to real life events, has also been successfully used to make serious points. The film 'Four Lions' is essentially a comedy about a group of home-grown terrorists planning an attack very similar to the real July 7 bombings in London. On the face of it this could just be an attempt to court controversy, but the film demonstrates that when done properly comedy can call attention to how insane a way of thinking can ultimately be.
The vision of terrorism laid out here is one of absurdity, an idea that conflicts with the norms of reality. Here the underpinning hatred is broken down into its elements and taken apart with its own contradictions.
Ultimately i think that the ability for a subject to be dealt with, analyzed and absorbed should extend to humor no matter how taboo the subject matter is considered. Humor can make make a very difficult subject easier to adsorb and break-down and carefully consider the absurdity of the motives that would lead men to fly planes into buildings. In an environment where difficult issues are being dealt with i think it is helpful- rather than harmful- to be able to make light of something.