I mean I don't think this is a universally American belief, but my point is that they're pledging their oath to the Constitution and the office of the presidency, but also swearing to defend against all enemies foreign and domestic. Basically the office is what you're swearing an oath to, the person holding that office is only there temporarily and that person can make themselves an enemy of both the title they hold and the Constitution that rules over all (or is supposed to in a sane world).

Think of it like when you have a substitute teacher or temporary manager, you're not going to hold them in as high of a regard as someone who's permanently holding that title or role because they're just the current placeholder who will be gone before long much like Trump.

And even if people in the military hold similar beliefs, they're not necessarily protected if they act on it because as our current president shows on a daily basis, the law only has teeth if the people in power want it to, so while someone may think they're acting within the bounds of the law, there's nothing to stop a corrupt, criminal administration from throwing the book at them anyway.