It'd be more appropriate to call them thieves rather than terrorists. Certainly that's what they are advocating, the theft of public lands for their own private purposes.
Yeah, thieves, squatters, illegal occupiers, blackmailers...there's lots of appropriate terms.
Terrorists is not one of them, because who the hell are these yahoos terrorizing? And if "holing up somewhere with a gun and daring the authorities" is enough to get someone classified as a terrorist, we're going to be getting Homeland involved in a lot of common criminal cases.