At a glance, in the states where Tesla can charge by the kWh, their rate seems awfully close to the average residential rate over the past year, plus maybe 1-2 cents.
Electricity Prices and Rates (Updated Jan 2017) – Electric Choice
I don't know enough about how commercial rates and demand charges work to say for sure, but my guess would be that, by pricing supercharging just above local residential rates, Tesla wants to achieves the following:
1. Earn enough on the spread between the commercial and residential rates to cover most of the upkeep of the facility (so it's neither a profit center nor a significant cost center)
2. Make "local supercharging" much less appealing for those with residential charging capability (e.g. home owners), but not prohibitively expensive for those who don't (e.g. apartment-complex dwellers)
All said, a smart and fair solution, if you ask me.
One thing I'm curious about: as more and more superchargers get hooked up to solar arrays, will the addition of free sun-power have any effect on Tesla's rates?