There is little incentive to attempt tyranny when the result can be predicted so easily.
Also, don’t underestimate the power of 100 million people wielding guns. The world has yet to ever witness a force 1/10th as great and well-armed as the American populous.
With a sparsely-armed populace, it's easy for the occupying force to roll through without much conflict or challenging decisions.
Winning occurs through attrition of the occupiers, which, unlike Vietnam, can't just "back out".