^^I looked into it some more and came across this:
http://www.theatlantic.com/international/archive/2013/04/photos-to-boston-from-kabul-with-love/275063/
It shows that according the photographer, her primary intention was to take the picture of herself with the sign. She then talked to the Afghans, who had heard the news and were concerned about it. They were asked to pose with the sign to show their heartfelt love.
Though I didn't think most of them knew about Boston at first, this is cool/heartfelt and "get well soon" card like.
If the story accompanied the pictures at first, it would clear things up. My first impression was that it came from the little girl (who had written the sign) and I do believe that if it came from the Afghans themselves it would have been more significant, but again still not many Afghans have the tools to do so (except their media).