Which part of “...Forts, Magazines, Arsenals, dock-Yards, and other needful Buildings” does grazing land fall under?
You do realize that in addition to docks and repair facilities that there is a Coast Guard auxiliary at Lake Mead? Also, as for “other needful buildings” how about Hover Dam? I would certainly consider Hover Dam a needful (power production for one) building. All of these are part of the Lake Mead National Recreation Area, land owned by the US Government that was at one time, leased to Bundy for his cattle to graze upon.
Then when Bundy stopped paying his grazing lease, he stopped having access rights to that land.