on what constitutional grounds to they continue to own it?
If no valid ground for ownership, they must sell it back to the states or private citizens.
Historical facts about the ownership of the land in question
Treaty of Guadalupe Hidalgo (1848 - Article II section 2 treaty) transfer ownership of the land from Mexico to the US
Nevada admitted as a state to the union (1864). Congress requires that the state ceded control of “unappropriated public lands” to the US Government. Nevada includes in their constitution this concession to the government.
The lands are part of what is called the Lake Mead National Recreation area and includes a number of buildings including dock-yards, and other needful buildings as listed in Article I section 8 under the enumerated powers of Congress.
If they don't OWN it, how can they SELL it?