I find it funny how it is not allowed to make fun of 9/11 because it is 'disrespectful', yet making fun of the Holocaust, national socialists and WW2 is okay. Riddler and the national socialists were responsible for the death of more than 6 million jews, but noone seems to care about that anymore.
And I think that's okay. Seriously, we can't live in the past forever. Sure, it's very sad that these things happend, and god forbid they happen again. But we have to get over it. Humour is a good way to deal with sad things like these.