I would hypothesize that the goal isn't preventing mass casualties, but rather preventing
mass casualties that shift popular sentiment toward making war with certain countries. (The government calls these acts "terrorism", but citizens being
afraid isn't quite what they have a problem with!)
The US government doesn't want a repeat of Sept 11. By which I don't mean "planes crashing into buildings"; but rather "operatives of some other country doing something on US soil, that results in US citizens becoming enraged-enough toward a particular other country, that — sensible realpolitik or not — any president that didn't declare war with that country, would be immediately impeached and replaced with one that would."
The US government, despite being a nominal democracy, wants to "the head to control the heart" when it comes to deciding when to go to war — because the heart is very easy to manipulate, and because there are always tons of realpolitik things going on that the heart knows nothing about.
I would guess that the government thinks that US citizens won't get too angry if terrorists bomb a bus, or a plane taking off/landing, or a line full of people. All these things are bad, but they don't strike me as being offensive to the American spirit.
The TSA, meanwhile, is very concerned about the airspace above... football stadiums. Which tells you about their thinking re: what would "offend the American spirit."