Indeed it is no laughing matter but what about titanic jokes? People talk about it all the time without consequence. 9/11 happened in the past and it's time to move on. I could argue that America had it coming for what they did in the Middle East but that's not needed.
The Titanic happened 100 years ago, almost everyone who experienced it is dead, not necessarily making it ok to joke about, but making it socially acceptable because people won't freak out about it.
The Holocaust happened 70 years ago, and people do joke about that, but it is considered dark and horrible.
Now personally, I think that a terrorist attack killing over 3000 civilian people thirteen years ago is still not ok to joke about. My father was in a position where it could have been him killed, too. However, I'm not telling you that you aren't allowed to joke about it, or the Titanic, or the Holocaust. I'm just saying that those kind of jokes don't really appeal to me. I find it disrespectful, but to you, it may not be. Just try to be mindful and respectful of others. That's all I ask.