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.
Which is why (in my understanding) ISIS does not do attacks on foreign soil. Their idea is to re-establish a caliphate (ei Islamic empire, which existed from around the time of Mohamed for centuries). ISIS would probably be okay with those that cannot join the jihad in Syria conducting a terrorist attack on their home turf, but would not actively plan one.
All I'm saying is I would not be surprised if this isn't specifically ISIS.