In a nutshell: "Those who cannot remember the past are condemned to repeat it."
While there are patriotic and nationalist undertones to these kinds of memorializing/commemorations, I don't see it as necessarily divisive as much as I see it as keeping the memory alive for posterity, lest we forget.
In other countries, there are also memorials and commemorations for less than fortunate events.
Someone already mentioned Japan above.
On the flip side of the same coin, China also commemorates the events culminating in the outbreak of the Second Sino-Japanese War (later part of WWII) as 國恥日 (literally: National Shame Day).
There's even a memorial bell with the inscription "Never Forget [Our] National Shame" in Shenyang.
The US is one of the few countries that has been fortunate enough to have only been invaded or directly attacked on its own soil less than a handful of times.
So, from that perspective, events like 9/11, may seem "small" compared to what some other countries have experienced, but in the context of American history, it is probably the most significant event of recent memory, and an event that still affects US policy and foreign relations today, nearly 17 years later.
So, definitely worth remembering.