It's much easier to mount a "resistance" with a few unregistered and unregulated guns in every household than not.

The US military is very good at wreaking havoc in places where you can just crusie-missle a square kilometer and not have any problems. That becomes difficult domestically: you can't just arbitrarily wipe your own cities off the map cause, you know, those cities are the ones who generate you income/GDP and have your actual population in them. There's no point in being a dictator of a wasteland.