9/11 was horrendous and a tragedy, but I don't care that people make jokes about it. There are equally, and countless more terrible things that have happened throughout the course of human history, and people freely make jokes about them without objections from anybody. Time goes on, and people will move away from sensitivity towards the events. People who make jokes about things like this, when you really get down to it, almost always acknowledge the true horror of what happened, but they don't want to focus on that. Most of the humor from this type of a joke comes from the irony; it's such a completely unfunny and morbid thing to think about, that the idea of making a joke out of it is funny in and of itself.