The answer then and now is, their allegiance is to the King. Or President or what ever he calls himself.
Apples and oranges.
By the time King George III sent British troops to quell the unruly American colonists, we'd already become a separate people. His soldiers weren't leveling their weapons at their own countrymen. By that time, we were 'countrymen' in name only - which is one of the chief reasons we became unruly to begin with.
An entirely different dynamic would be at work if American service personnel were ordered to level their guns at American citizens. The first problems the military would encounter, would be mass desertions and mutiny in the ranks. Maybe even fragging of officers.
Those who would obey such treasonous orders, would be facing insurmountable odds against nearly a third of the American population, who would be enraged, and armed to the teeth.
The odds of prevailing militarily would be hundreds of times worse than both the Iraq and Afghanistan wars put together. A total no win scenario for the traitors and their 'King'.
You are not factoring in the goal of destroying the United States.
The Desolation of Obamanation has no interest whatsoever in either side prevailing.
Only that we are swept off the world stage and left a scorched ruin.