An armed populace creates a huge risk for a federal paramilitary force descending on a municipality with the intent to terrorize the citizens. They're not rolling in with tomahawks and tanks, they're coming in with assault rifles and window breakers.

It won't "stop" them but having to treat everyone like they might shoot back and show up with a 10:1 manpower advantage and armed to the teeth every time you wanna subject someone to state violence really puts a damper on your ability to do tyrannical government things.

The problem with that thinking is that you have to have the will to act to stop tyranny, and no amount of armament will give you the will or the foresight to see it.