This cartoon is so good exactly because it is controversial. It gets people talking and forces us to take time to think about a hard subject. Before people get worked up and comment on an image this provocative, they need to take some time to think deeply about the artist was trying to portray. Not all cartoons are meant to laugh at. They can be thought provoking too. The passing of 3000 or so innocent people is tragic, but I think everyone, and especially Americans, need to take a seriously look at what happened that day, beyond the stories on CNN.
Consider this, over 100,000 innocent civilians have been killed in Iraq alone since America started this atrocious war ([link]). One hundred thousand. Why is this not all over the American news? Why does this not sicken every American citizen as they were on 9/11? The idea of a national identity is a fallacy. Nations are imaginary lines drawn on one planet that is occupied by a fragile human species. There is no difference between your neighbour across the street and your neighbour across the ocean. The sooner we wake up from this fantasy the better off each and every one of us will be.
I believe it entirely you would see this some other way if your family members or your relatives would have been lost into the attack. Towards us this is called dead person vilification, you towards you for "resistance".