That's the point. If you can laugh at either without feeling bad, you ought to be able to laugh at both without feeling bad. In both, thousands of people died. Both were huge tragedies. If you feel bad about 9/11, you ought to feel bad about the titanic, but people don't. People feel guilty about laughing at 9/11, but not so much at older things like titanic, pearl harbor, etc. Time passing leads the most recent horrific thing to be the most meaningful, for instance Holocaust>Inquisition. Why? Holocaust survivors are still alive. No one is alive who remembers the inquisition. Both were horrible, but one is more meaningful because it is closer to our own time. It's rather interesting to think about how time plays into the way we perceive tragedies.