This particular operation is either ISIS-conducted or ISIS-oriented vigilanteism; whichever it is, backing down in Syria will only embolden them. (Or rather, embolden those like them; I don't imagine that very many of the specific attackers here are going to have particularly many opportunities to do this again in the future.)
ISIS is specifically out for either world empire or apocalyptic defeat (see http://www.theatlantic.com/magazine/archive/2015/03/what-isi... ); tactical concessions will work about as well as they did with the Nazis and the Communists -- or even less well than that, since neither Naziism nor Communism believed that success was a sign that Divine Providence was smiling on them.