I do not know the answer to that question, tmoz.
Common sense from an accountant's perspective:
If a business is going to invest heavily in technical infrastructure like a Supercharger (and more than likely canopies with solar panels atop), it makes much more sense to purchase the land outright rather than enter into a long-term lease. I would think that real estate in Quartzsite, Arizona would be rather cheap, probably a lot cheaper than the total cost of construction.
If leased, the landlord can boot Tesla at the end of the lease, or worse, buy them out if that real estate unexpectedly skyrockets in value before the end of the lease.
Fee simple ownership also makes it much more attractive down the road if Tesla were to spin off its Supercharger network as a separate company. The presumed buyer(s) of this network would not have to worry about the value of the assets acquired plummeting as leases expired if renewal were unlikely.