There's a reason the CMS and Santana Row superchargers are busiest during mealtimes and on the weekends, the mentality of many is we'll "charge while we eat", not we'll go "eat while we charge" as if on a road trip. Meaning, you'll see a lot of families getting out of the car to have a meal or snacks, walk around, they're going to a destination, and maybe the supercharger factored into the choice of locale. But if the supercharger is full, the cars don't line up or leave for the next supercharger - they park in a regular stall and go eat as they planned to; getting a full charge is a secondary concern.
Also not mentioned in this thread is that Santana Row has a couple of other choices - valet Supercharging and free Tesla destination chargers (a lot of them). One could ask a similar question, why pay $8-10 for valet on top of your Supercharging fee, when you can drive up the ramp and NOT pay the $8-10. Or if it's all about price, why not use the free destination charger at an effective 100% discount? To me, it's the same answer, they're there to "charge while they eat", not the other way around.
And speaking of eating, a family of four dining out there is going to be spending $100-200 on top of the electricity needed to get there and home. Plus maybe more for shopping, dessert or drinks. It's the family destination time - a $12 difference in charging cost is just a convenience fee for the entertainment. Just like the huge Valley Fair mall next door has no supercharging at all, but just started charging for its parking structures - is anyone NOT going to go there or park there because it'll be a few bucks more than at the other shopping centers nearby? No, most are still going to park and shop there regardless.
