25 years ago, 9/11 was carried out by terrorists. Not because of "revenge," not as a "terrible but predictable reaction to oppression in the Middle East," but because of the desire to spread Islamism. Osama Bin Laden expressly stated this in his murder manifesto.
"Never forget" doesn't just mean never forget that 9/11 happened. It means never forgetting why it happened.
The reason is not cosmic justice or an act of retribution. It was a direct, unflinching attack on a world that threatens everything the killers stood for.
A world that represents freedom, individuality, democracy, truth, epistemic sovereignty, and the pursuit of happiness. All the things that prevent their vision of tyranny and subjugation.
That's what they tried to attack 25 years ago. The goal was not just to kill people in the USA that morning. It was to kill the USA in the minds of its people.
"Never forget" means never letting that happen. It means protecting the dream, and never seeing the land of liberty through the eyes of its adversaries.