What happened after 9/11 was a shameful period in American history, one that we're still at the tail end of, as witnessed by the Tea Party. However, even in the rottenness of that time period there were still valid points raised. One of them was the rebirth of a sense of American values, of unique ones, and the criticism of folks who opposed this idea as being part of the 'blame America crowd'. The United States has done a lot of messed up stuff around the world, and continues to do it, however there are also positive features of American society and culture that do exist, and there's some truth to the notion that these things got a little bit of short shrift in the media before 9/11, although this is in some sense exaggerated. But, on the whole, true enough.
The response to 9/11, though, was completely wrong, and many of those values were either superficially understood and mouthed as slogans, or not understood at all, and instead used to justify whatever revenge fantasy that the person uttering them had at the moment. These values were used to justify authoritarianism, Christian theocracy, racism against people of color who were either Muslims or who seemed to be them, discrimination against Muslims as a whole, the creation of a false 'culture war' between the West and the Muslim world, two invasions, one of which had little justification, the other having no justification whatsoever, Guantanamo Bay, extraordinary rendition, the list goes on.
What could have been at least somewhat noble, with a rediscovery of the positive within American society---not to the exclusion of what we do wrong, and have done wrong in the past, but put side by side with it--turned into a nightmare, with things such as the Flag being turned into symbols of oppression, that people in general had to respect--or else.
Some of the criticism, that boiled down to things like not feeling comfortable in saying that you're proud to be an American before 9/11, may have had some truth....but the solution to that isn't to completely discount the negative things that America has done wrong, but to honor the positive side by side with it...and attempt to stop doing the negative things in the first place and possibly right some of those wrongs that have been committed.