We here tend to work more of a spirit of freak-outery and cynicism. And as much as it annoys me, I think its more a symptom of how relatively little most of us and our ancestors have had to worry about in the US as much of our history was as a mostly rural international backwater that quickly transitioned quickly to a world power with oceans and, almost always, peaceful neighbors on its borders.
Which is something to be thankful for, even if it has led to a fairly weak resolve.
But that is just me making up a hypothesis that fits the data in retrospect. Maybe something else is going on.
I don't remember much about NYCs reaction, as I was in school in Atlanta at the time. But my father worked on an air force base at the time and reactions there and around that town were anything but carrying on.
Large chunks of the London population would have also lived through the various IRA campaigns in the 70-90s.
One of the thing that seemed darkly humorous to me was that in the aftermath of the July bombings the tube/rail services rolled out the old warning messages used during the IRA campaigns which didn't really apply to suicide bombings (they were all aimed at suspect packages being left somewhere - rather than things being carried / suspect behaviour).