Don’t these people realize that the military isn’t supposed to be used inside the USA?
No, their model for what the military ought to be is the Chinese Red Army.
That would be news to General Sherman and a host of others. Including MacArthur, Patton, Eisenhower, and Washington. There is absolutely nothing in Constitution to prevent it. Washington, who surely knew not only what was in the Constitution, but what it meant, led federal troops, and federalized militia, in numbers rivaling the size of the entire Continental Army during the late Revolution, to put down the Whiskey Rebellion. Directly under Washington were Alexander Hamilton and "Light Horse Harry" Lee.
Later Marines and soldiers, under the leadership of then Colonel Robert E. Lee, were used against John Brown and his band of abolitionists at Harper's Ferry Virginia (Now West Virgina).