I think the reason they are charging per fill-up rather than an up front flat-rate is to try and move the people charging to their house. If you already paid for something than you are going to want to take advantage of it. On 3-Day while we waited in-line for the store to open a stream of Teslas appeared after dawn trying to charge.
"Sure I could have charged at home, but here it only takes minutes and I am done for the week..."
This means that those who arrive and don't live near will have to wait even though there are lots of chargers. If you have to pay each time, even though it will likely be cheaper for years than paying a flat-rate up front, it will make you think of the OTHER things you could spend that money on and you'll plug in your charger the night before. Good boy!
I still would like it if they charged more for supercharging near home and less when away, that would emphasize the purpose of the supercharging network is to remove the range anxiety, not to replace home charging.
Not sure how to fix this for the free-charging Volta network. Those chargers are in use for days!
-Randy