This is a manhole cover not the twin towers. While jokes may be inappropriate they don't cause harm or make the accident worse, they are simply jokes. Another big difference between this incident and 9/11 is that it happened in SF, which a lot of people simply do not like and they forget other people live there. I do not think making jokes does any damage or makes things better, they are just jokes.
I thought the Dems had a lock on being offended, but I guess we have our share too. If the jokes offend you maybe you should not be on this thread.