Not a constitutional scholar here, by any means, but I thought there were only two ways the Feds could own land.
Either for armories, government offices, and military bases, or if the property owner was compensated under the "Takings Act." Neither applies to the land the Bundy cows grazed on, from what I know. (Articles I and V, respectively)
Many now question whether the Fed agencies, such as the blm and forest service, can exist constitutionally.
I think people just put up with it until the Feds pushed too many, too far.
So, this is a good thing that needed to happen.