Ever seen a rat? Some folks live and work in circumstances where they will rarely, if ever, see one. And when they do, it is the shock of their life. heck, they may even have nightmares about it. Other folks, because of where they live or work, or both, have enough encounters that their reaction is "Aw, geez, another one? Better leave the cake in the fridge. Martha, can you go get the Warfarin box?".
While it has seen too many of its own lose their lives in conficts overseas, the United States is blessed with having had precious little warfare (apart from Pearl Harbour) within its own borders, in living memory. The same cannot be said for most other industrialized nations, where monuments to this and that, and the preserved ruins of this or that, can be found all over. Thankfully, humans have toned down their act in many of those places, such that the experience of war is largely 2nd hand, and history-book stuff, for many. But folks remember.
The shock that you reflect, is, in many respects, a byproduct of what it contrasts with: continued peace. That's not any sort of diminuition of the events that prompted Amanda's thread, or any sort of delegitimization of the reaction that many have, but a partial explanation of its depth. People react strongly to things they've never had to live through before. And in the absence of war on domestic soil, people react to events like the Boston bombing, or Newtown, or Oklahoma City, or the Twin Towers, as if it were war. Because, apart from those that served in the military, or emigrated to these shores from places torn apart by constant war, these events are the closest many have ever come to war.