Except it's not a service it's an added feature on the car that they disable and enable via software like many other features on the car.
If I subscribe to satellite radio and I sell my car I'm not paying your monthly subscription I'm paying mine. That subscription follows me even if the car is equip for that radio.
The supposed "subscription" to auto pilot as you put it doesn't follow anyone anywhere. If you buy another Tesla that service as you call it isn't included you'll have to pay for it again meanwhile if you sell your old Tesla they disable it.
The duration of the subscription is your length of ownership of the car.