I get your drift and point, but they don’t have the sworn duty to uphold the good welfare, wellbeing and safety of the force .. the military leadership ranks do.
The "posties" lost that when they listened to this guy, or stories about him, and said nothing.
None of the good Major Hassan stuff showed up in regular stories, but they went after the commanding officers like they were monsters.
There's reason to believe the "posties" (all of them or some of them) KNEW or had REASON TO KNOW that this guy intended to kill people.
It is not unreasonable to believe that they, themselves, were part of a conspiracy to assist.