Perhaps you could unpack your reasoning a little?
I’m not seeing anything in Article 1 Section 8 that allows the government to assert private property rights on grazing land.
Unless the whole area is one enormous post office :0)
The land in question is a small portion of what is now known as the Lake Mead National Recreational Area. Even so, the land in question is over 600,000 acres. That is roughly the size of half of Delaware.
So Article 1 Section 8 says that Congress has exclusive legislative authory over lands that the government owns. If you want to look into the qualifying clause, Hover Dam (Lake Mead National Recreational Area) certainly qualifies a “... other useful building”