To those saying "Big deal, thousands die every day":
Just because lots of people die doesn't mean we can't remember the first time in history in which an attack on the contiguous United States was carried out by foreign nationals. 200+ years of no attacks is better than any other country, given the number of wars we've been in. That alone merits it's being mentioned.
Also, stop saying "get over it". I got over it in 2001, like most people. What people are doing is trying to honor the victims' deaths.
On top of that, we reserve the right to feel bad about a specific group of people dying, even though worse happens elsewhere. You're just as bad as the "CHILDREN ARE STARVING IN ETHIOPIA" people.