It's cheap, but currently there are 649 supercharger stations, and 3906 superchargers[1], and he gives that power away for free to Tesla owners. Buy cheap sell free doesn't really scale, unless it's subsidized in some other way (and reducing cost even more may make that more palatable).
There's also the powerwall play. I imagine a lot of people that signed up for powerwall will want solar at the same time, if they don't have it. Maybe this is actually a way to hedge against powerwall coming in at cost (or a way to get an installation network). If most the installs come with some solar installation as well, maybe they can see the device at a loss and meet cost expectations while still not losing money.
There's also the powerwall play. I imagine a lot of people that signed up for powerwall will want solar at the same time, if they don't have it. Maybe this is actually a way to hedge against powerwall coming in at cost (or a way to get an installation network). If most the installs come with some solar installation as well, maybe they can see the device at a loss and meet cost expectations while still not losing money.
1: https://www.teslamotors.com/supercharger