It used to be rational, rather. The classic terrorists of the '70s wanted "a lot of people talking but not a lot of people dead" -- and also wanted to survive the experience. The new breed of terrorism in the '80s and '90s was different from those; the IRA, the PLO, and various state-sponsored terrorists in that period had/have more in common with each other, despite their obvious differences, than any have with al-Qaida and ISIS, which aim for suicide missions with high body counts.
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.