Adding to this, if you're in the peacetime military and have a long contract, and congress or the executive orders the military to deploy somewhere, those soldiers won't even have much of a choice. As long as you're under contract, if the military says jump, most often all you can respond with is "how high?"
I also recognize that the US has perverted this concept countless times since with blatant imperialistic motives, it does however still not break the concept of a just responsive military force.
TL;DR: They're not selling their souls if they (mistakenly) trust elected officials to work for the will of the people and only deploy for just causes.
Fortunately the Nuremberg principles rescue us here. Following orders is no excuse for abdicating a moral responsibility.