Jonathan Jones in The Guardian:
In the photograph Thomas Hoepker took on 11 September 2001, a group of New Yorkers sit chatting in the sun in a park in Brooklyn. Behind them, across brilliant blue water, in an azure sky, a terrible cloud of smoke and dust rises above lower Manhattan from the place where two towers were struck by hijacked airliners this same morning and have collapsed, killing, by fire, smoke, falling or jumping or crushing and tearing and fragmentation in the buildings' final fall, nearly 3,000 people.
Ten years on, this is becoming one of the iconic photographs of 9/11, yet its history is strange and tortuous. Hoepker, a senior figure in the renowned Magnum photographers' co-operative, chose not to publish it in 2001 and to exclude it from a book of Magnum pictures of that horribly unequalled day. Only in 2006, on the fifth anniversary of the attacks, did it appear in a book, and then it caused instant controversy. The critic and columnist Frank Rich wrote about it in the New York Times. He saw in this undeniably troubling picture an allegory of America's failure to learn any deep lessons from that tragic day, to change or reform as a nation: “The young people in Mr Hoepker's photo aren't necessarily callous. They're just American.”
In other words, a country that believes in moving on they have already moved on, enjoying the sun in spite of the scene of mass carnage that scars the fine day.