Or at least it was, by some of the oldest laws known to humanity--- Right of discovery and conquest. From the moment we first planted an American boot on Lunar soil.
Until our government essentially it away.
"Thief! Thief! The American wants to steal the Moon!" Oh shut up. First time ever that we land on territory that doesn't have natives to wring your hands over or trees to hug, and you're gonna squeal that we wanted to claim it? Typical. You won't let us claim an airless, lifeless rock out of sheer spite. So our milksop government folds like wet toast and makes some blithering proclamation about how the Moon "belongs to the nations of the world"--- despite it being American scientists, technology, and astronauts that made the trip possible. Thereby guaranteeing that nobody would EVER go back.
We should have told y'all to go pound sand. The usual crowd of French popcorn farts would have squealed about American Imperialism (over a BARREN AIRLESS ROCK no less), and Kennedy should have looked them in the eye and said "You want to stake a claim there? Come and get it." And given presidential orders for a permanent lunar military base. Wouldn't have to be much: just a little space hut occupied continually by one or two men.
Within the week every country on the planet that could screw a nut onto a bolt would be building a lunar colony rocket, just to spite us. We and every other country on the planet would have mining colonies on the Moon today, and we'd be halfway to settling Mars, if we had just treated the Lunar territory like every other territory on the planet: first come, first served.
Instead we gave it to the biggest collection of do-nothing weenies on Planet Earth--- the United Nations. And the Final Frontier died on the pagan altar of collectivism, at the hands of the Tragedy of the Commons.
We could still do it today. We SHOULD still do it today. The Space Race would take off the next day.
But of course not. That would be too CAPITALIST.