Bundy family established rights to that land through blood an sweat, prior to ANY legal authority.
As a state and county authority began to emerge they codified water, grazing and access rights.
Then came BLM. Changed all the rules and let them know there is a new master in town.
Bundy contends his rights both predate and supersede the Feds/BLM.
Legal or not, it's not an unreasonable position.
Working land does not give you title to it. The new area he has been grazing is an area he did not use prior to losing his 1998 court case. From the 50s on, he accepted the BLM’s authority to set grazing limits. If you spend decades paying someone to use their land, it is hard to then claim you own the land because you have been using it!
And remember, he first rejected paying the BLM after 12 years of Republican Presidents had managed the BLM, including 8 years of Ronald Reagan.
If he had argued water rights or some other principle, he might have made a case. But arguing that the federal government didn’t own the land, when the Nevada State Constitution specifically says they do, is a tough sale to a court...
Perhaps Bundy sincerely believes that he has a right to use some of that land without interference but that doesn’t make it legal.
BLM/government management of those parcels is not something that recently happened and his family seemed fine with paying their fees until around 1993, so by precedent he acknowledges that the BLM and federal government control that land and are entitled to fees.
In addition he apparently has expanded his grazing to areas that his family had no historical use of. As soon as he did that, his whole “history” argument goes out the window. BLM is not innocent in this whole matter but it seems to me that Bundy and his family are opportunists who are looking for ways to get something for free and throwing excuse after excuse against the wall to see what sticks and gain sympathy and backing from people who may have other legitimate unrelated gripes about the government.
Unfortunately if Bundy ends up being a scammer, everybody who associates with him and his “cause” will end up painted with the same brush.