Re: potential for future attacks, the security sectors are much stronger now than they were before 9/11. Also, AQ didn't do itself any favors with indiscriminate attacks that killed a lot of Muslims (for example the 2005 hotel attacks in Amman, Jordan). That lost them a lot of support. This isn't to say that future attacks won't take place.....they probably will. Just that AQ as an organization has been significantly weakened ....and the security sector strengthened...as compared to the pre-9/11 timeframe.
As for the other matter being discussed (and with a little trepidation on stepping into roiled waters):
I'm willing to cut some/many of those celebrating some slack. Many of them look to be of college age; presumably cooped up in libraries and dorm rooms studying for finals (it *is* that time of year....).
College students in general actively look for things to party about, especially around finals times. For an illustration, see some of the videos of university flash mobs that are posted to youtube. Its a positive distraction; a way for them to have some fun and let off some steam...
Nor are college students known for their ability to be tastefully nuanced on issues like this. They are, after all, quite young. For those with protect and serve mentalities....and those who want to enter the security sector in particular....its been a tough 10 years. The US has been heavily criticized for many of its post-9/11 policies and there are few easy 'wins' to point to where they can say they are proud to be American. And that, more or less, is what many of them want to say...
Given the context (death and destruction) perhaps it might have been more tasteful if some of this celebrating had been quieter, but I do have to say that it was fun to watch the younger folks express their pride in being American...