They're not especially threatened by citizens permitted to own firearms either. Do you really think that the average gun-owning homeowner can go toe to toe with a soldier who has armor support, air superiority, years of training, and, most importantly, modern military logistics backing him up?
And before you bring up insurgencies, let me remind you that successful insurgencies are marked by two features: 1. the occupying army does not speak the same language as the locals, so intimidating translators is an easy and effective way to deny the occupier situational awareness. 2. There is a bordering country willing to support the insurgency with either arms, safe haven, or both. Neither point applies to the US homeland.
Arguably, the last time an insurgency tried to take on the US Army was during the Civil War. Even though both sides had similar quality armament (and the South had higher quality leadership), the North won, thanks to its much stronger industrial base and better developed logistical system (i.e. railroads). The situation is even worse today. The average homeowner does not have easy access to any armament substantially note powerful than an AR-15, and the chances of any branch of the federal military defecting to support a civilian revolution are vanishingly small. In addition, the military speaks the same English that we do, and I'm pretty sure that neither Canada nor Mexico is willing to furnish you with arms and/or IEDs. So, with all that in mind, would you care to explain how, exactly, the Second Amendment protects me from the US government becoming a tyranny?