There are potentially other ways to monetize the Supercharger network besides charging Tesla owners for charging. Discussed on TMC in detail years ago, 2013ish. Might be worth a search, I can't find it right now.
Early in the Supercharger Network's life (when there were 6 or so total chargers, all in CA) Tesla indicated that they have a very predictable usage pattern. Peak use Friday afternoon, when people leave for weekend trips, and Sunday when they return. Most of the rest of the time, they sit idle/near idle. Tesla is also going to be installing batteries at Supercharger sites, to help deal with peak usage demand charges from the utilities. But, if the sites sit mostly idle, then the batteries aren't needed for demand peak shaving related to charging, and can provide other services to the utilities. If Tesla puts stationary storage at Supercharger sites, and usage patterns remain similar, they will have lots of grid-connected storage that is sitting idle at peak grid usage times. So they could sell services to utilities to pay for free charging. Tesla energy has been kicking around for a long time, and seems built into the company's ecosystem.