I don't quite grasp how it could be offensive.
Well, there are a few reasons I could think of, based on my knowledge of how the USian mind works. (I should point out that I do not find the ad to be offensive, but think it is brilliant)
1. USians have a kind of apocalyptic reverence for the attacks on the World Trade Center. They think it's a sacrilege to refer to the event or use the image of the Twin Towers in any kind of advertising - even for a non-profit, non-commercial, or charitable purpose. It's not unlike the way some Muslims reacted to the Danish cartoons.
2. USians believe 9/11 was an unspeakable horror of incomparable proportions - largely because they are ignorant of the massive amount of death and suffering that goes on every day around the world, much of it caused by their own government's military and economic policies. Also because they don't regard the lives of brown and non-Christian people who live far away to be of comparable value to their own lives or the lives of their fellow Americans. Consequently, they hate to hear anybody tell them that, for example, as many people die every 26 days on US roads as died in the terrorist bombings of 9/11, or that the 2004 tsunami killed 100 times as many people as the 9/11 terrorists, or that every two weeks the casualties in the iraq and afghanistan wars exceed the death toll on 9/11, etc.
3. USians (outside of New Orleans) believe they are personally more at risk from terrorist attacks than from tsunamis or other natural disasters. They think it trivializes terrorism to compare it with natural disasters.
4. Deaths caused by terrorism are worse than deaths caused by tsunamis, because tsunamis are caused by God. Terrorists=evil; God=good.