Ranching in Nevada

From The Blaze:

Armed Fed Agents and Snipers? Nevada Rancher Is Taking on the Gov�t in a Battle That�s Reaching a Breaking Point
Armed federal agents deployed last week to northeast Clark County, Nev., for what can only be described as a major escalation in a decades-long standoff between a local cattle rancher and the U.S. government.

Cliven Bundy, the last remaining rancher in the southern Nevada county, stands in defiance of a 2013 court order demanding that he remove his cattle from public land managed by the U.S. Department of the Interior�s Bureau of Land Management.

The 67-year-old veteran rancher, who has compared the situation to similar confrontations with government officials in Ruby Ridge and Waco, Texas, told TheBlaze that his family has used land in the 600,000-acre Gold Butte area since the late 1800s.

�I have raised cattle on that land, which is public land for the people of Clark County, all my life. Why I raise cattle there and why I can raise cattle there is because I have preemptive rights,� he said, explaining that among them is the right to forage.
�Who is the trespasser here? Who is the trespasser on this land? Is the United States trespassing on Clark County, Nevada, land? Or is it Cliven Bundy who is trespassing on Clark County, Nevada, land? Who�s the trespasser?�
Claiming that all other options have been exhausted, the Bureau of Land Management and the U.S. National Park Service responded to Bundy�s inflexibility on the issue by calling on federal agents and contract cowboys to restrict access to the public land and to confiscate Bundy�s �trespass cattle.�

My money would be on Bundy -- his use has been grandfathered in since the 1800's. Why does the federal government think it can come in and just take the land?