It has been interpreted as yet another example of indifference or the compulsion to return to normal even though, as anyone can see, there is nothing normal about what is happening. It is the emblematic photo of our times.
Photography, of course, is often a lie, and this photo is no exception. It captured a moment, a second or less, when one of the subjects said something and the other four turned toward him and away from the plumes of smoke, so they seemed not to care. This photo, like all photos, lacked context -- what went before and what went after -- and the interpretation of insouciance has been challenged by no less than some of the people in it. They insist they were intensely aware and horrified by what happened.
Exactly. I'm glad someone is standing up for those kids. I'm sure, during that day (I was in DC), there were plenty of moments where I smiled at something, or did something other than look horrified and shocked (which I was). Hell, I went for a drink with my roommate and some friends. There would have been thousands of opportunities to take a picture of us to say that we were being insufficiently sensitive.
But it is the delight we take in taking things out of context, in cherry-picking our quotes, in not understanding the context. So much easier never to scratch the surface than to actually stop and think.