How, in any possible way, is this constitutional?
It's probably utterly aboveboard and constitutional in that when the federal government gave the land to the state, it was with the understanding that the state would maintain it as public parks.
Well, the state's probably going to renege on the agreement, closing them, which means the federal government can take back its "gift."
I don't think the federal government, if it took the parks back, is going to let them lay fallow; I'll bet they operate as federal parks, which does nothing to ease any burden on the taxpayers of the other 49 states.
After which "federalization" of this-and-that in California, until the taxpayers of the other 49 states are keeping California afloat.