Actually, it is not because its mere existence as de facto Federal lands without a need to fulfill Constitutional duties is clearly unlawful. The patriots in Nevada and Oregon are doing a great service by focusing national attention on this seldom considered issue.
The solution would be for Utah and Oregon to buy the land. Congress has the ability to do so.
It is Federal land de jure as well as de facto. In other words, it's just Federal land, period.
What is "the need to fulfill Constitutional duties" exactly?
The Federal government has the ability to acquire land and has been doing it since the first days of the republic.
There is no list of "duties" that need to be "fulfilled" in order legally purchase land. You purchase it.