This. An insurgency within the boundaries of the US could strike at key infrastructure and it would be very hard for the police and military to protect everything across such a huge country. If insurgence caused serious disruption at one or two critical factories, or shut down a port, or took out an important rail bridge they could cripple the country.

Look at how much damage the Mohawk did to the Canadians by just blockading rail lines and preventing rail traffic. They didn't even have to attack any actual people but they caused massive disruption until Klanada mobilized the army and sent tanks to chase them off.