According to the clause in Article I, Section 8, of the United States Constitution, there are certain criteria required to be met in order for the federal government to own land within a State
The issue in Oregon, where an armed militia is standing up to the federal government, as they did at the Bundy Ranch in Nevada, isn't as much about ranchers burning a fire line on the edge of their property to protect their own property (and it getting away from them and burning into "federal land"), whether or not the Hammond family even wants the "armed militia" there in their town, or that the "armed" militia is willing to occupy a ranch on the edge of federal space in the first place.
I don't believe, as the media and some of the police does, that these militia members want to overthrow the county and federal government hoping for some kind of nationwide "blood in the streets" coup. It is really all about government authority, and if the federal government is functioning within the "realm of authorities granted by the Constitution."