Part of it is that we're armed. Part of it is that they do NOT want to shift public opinion in the US into a generalized hatred of Muslims. They want to intimidate the government into adopting certain measures, using the threat of more attacks.
Visualize the effect on the Muslim world if the US populace started to hate Muslims in general, to the level where if Kenyans started killing Muslims, the general reaction here might be "Good! They had it coming!"
If the US had not intervened in Bosnia during the Clinton years, the Serbs might just have done an ethnic cleansing. The only thing stopping such in some parts of the world, is concern that the US would be very displeased.
A side effect of Obama's running down the US is that pretty soon no one will care what the US government thinks. It's becoming that way both internationally and domestically.
Islam should be more familiar to moviegoers as a form of Zombie takeover. There's no compromising with the Undead.
An old friend joked after 9-11 “Well, I guess it is time to play cowboys and muslims.”
The US can be the most ruthless country out there, once it becomes a matter of survival. We destroyed the German state, ended (in large part) the old Japanese civilization. Once we decide to do it, we can smash the world of Islam.