I think they do it on trade ins, too. If you read the FAQs on the premium connectivity, it specifically states that if a used tesla is bought from tesla, they may remove the premium connectivity even if the person trading it in had lifetime (paraphrasing).
If purchased private party, whatever software is there stays there. If someone trades in a tesla to tesla with EAP, tesla can (and likely would) remove it.
(premium connectivity is where I got this, but it likely shows teslas thinking on software in their cars)
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How are used Tesla cars affected by this policy?
All used Tesla cars purchased on or before June 30, 2018 will continue to have the Premium Connectivity features described above at no cost for the lifetime of the car, and will be active for future owners i
f sold privately.
What happens to my Premium Connectivity subscription if I sell or trade-in my car?
If Premium Connectivity is included on your car at no cost, it will be removed automatically at the time of trade-in. If the car is sold privately, Premium Connectivity will remain for future owners for the lifetime of the car.
If you’re subscribed to Premium Connectivity and sell your car privately, you will need to log into your Tesla Account to cancel your subscription. If you trade-in your car, your subscription will be canceled automatically at the time of trade-in.