Not enough Americans understand why there are wars. Too many just blindly wave at the flag and say that any war has their full support.
Wars usually have a couple things in common at least - someone who thinks that they either have an advantage and something to gain, or are threatened, and flase justifications for the war.
Take the American civil war. The south had years and years of grievances, with no small amount of justification, that they were being screwed by the democratic principle that the majority non-southern states could screw them every which way economically, and they could only be the 'loyal minority' with so much abuse. The North, of course, arranged for the south to shoot first to look like the good guys - and the war became about ending slavery. Later.
Or take the previous war to that, the Mexican-American war. You had the US feeling its oats, and getting enoug more powerful than Mexico, who had a lot of land, that it encouraged the US to take the land by invasion. To do it, the US president unilaterally declared that the border had moved, put a few troops on the new 'border', and waited for a skirmish. Between that and the Alamo, the US were the good guys again.
Of course, not all Americans fall for the propaganda, but enough do. For example, Lincoln was a leading opponent of the war, and Grant called it as immoral war of a greater power by a gereater power against a weaker as there ever was. But the US smiled and kept the land.
The US, in my opinion, did wrong to many in the Middle East in several situations, for which 9/11 can be seen as a response - as or more justified than the American attack on Afghanistan in response to that.
Basically, it goes back to oil - the world industrial powers a century ago screwing the middle east to set things up for their benefit, and continuing to screw Middle Easterners, set up puppet regimes, and so on, in ways which leave them with little to say when they are given a tiny, tiny taste of their own medicine, relatively speaking.
When only the sanctions on Iraq, for just one example, killed hundreds of thousands, mostly children, horribly through malnutrition and disease in a previously prosperous society for the region, it outraged the region, and many of us Americans, who felt that the policy could be greatly improved in terms of reducing the harm to innocent people. Those hundreds of thousands greatly outnumber 9/11 victims.
That doesn't make 9/11 "right", any more than it's "right" for the Hatfields to kill a McCoy after the McCoys kill some Hatfields, but the thing is, too many fail to avoid the arrogance of power, and could care less about the suffering the US causes with its abuse of power over and over.
I don't think Americans should excuse 9/11; I think they should take responsibility for their own government's wrongs preceding 9/11. 9/11 was also a publicity stunt, a power grab, for Al Queda, serving their interests; like any power, they used the wrongs of the US as justification, just as the US used, with less justification, the Alamo for justification in its immoral war on Mexico, or Custer's Last Stand for its immoral war on the Native Americans.