It's free. If you could get gasoline for 200 miles for free, would you mind waiting for 30 minutes? I guess it's $25-30 worth of gasoline, corresponding to $50-60 per hour of waiting. I could wait.
With the battery swap that Tesla is now introducing, you could also chose to swap the battery in a couple of minutes.
So it's built into the price of the Model S and TANSTAAFL and all that. The point being that you don't do a transaction at the supercharger station in the same way you do at a gas station. Theoretically, you could use a supercharger station multiple times a day and pay nothing additional.
It's like the "free maintenance" on BMWs. Or "free loaner cars" when your car is in for service.
Seems pretty nitpicky to point out that it's not in fact free. Whatever it is, it's quite a bit different than chucking some dollars at Chevron or whomever.
With the battery swap that Tesla is now introducing, you could also chose to swap the battery in a couple of minutes.