these were most likely filmed on federal land that is super easy to get permits for. there is a large area in inland southern california for these types of things that the federal government controls and is legally required to give permits for (unless environmental concerns like leaving your trash arises).

if this is true, its the same area where burning man and other desert festivals are held : D

Green screen technology was too expensive. Relatively easy way to put the stars in a big space versus dressing a giant soundstage.