The car passing through Tesla ownership might have something to do with it.
I put a deposit on one from the used Tesla on line inventory, Tesla owned, four days before they discontinued the free supercharging, what it said in the ad was free unlimited supercharging for life. I had no idea that they had discontinued it after I bought it, and when I picked the car up a week or so later and paid the balance I couldn't figure out why the guy delivering the car was going on about the free supercharging, and how lucky I was, and, who knows if he knew what he was talking about, but he said that Tesla could not take it away if I sold it unless the car went back through them, they would be devaluing the car for me by removing an option that I in theory paid for.
My option code says SC01.
It made sense a couple of weeks later, I was looking for something else and ended up on the used car page, saw that none of them had free supercharging, wondered why, Googled it.
Anyway, mine does have it for me, and if this guy knew what he was talking about it's transferable. Caveat is that he also told me I couldn't upgrade to 4G, which is not true.