Which one didn't pay for FSD:
1) Original Owner
2) United Trader, the auction house
3) Alec the current owner
Tesla did produce a copy of the original Monroney sticker that indicated that FSD was paid for. It did not provide any proof that the original owner skipped any payment mentioned on the sticker.
When United Trader bought it from Tesla, FSD was there physically and functionally driven by the auction house. Tesla did not produce any document stating that United Trader did not buy FSD from Tesla. Tesla did produce a detailed list of what was done / taken away, repaired with the car but that did not include FSD removal.
Alec the current owner believed the United Trader's story that it did come with FSD because they drove it themselves so he agreed on the price he paid for the paper trail that he got: nowhere there are any mentions of FSD removal.
Not until he got the news from Tesla Service Center that he, Alec, the current owner didn't pay and Tesla never mentioned anything about no payment from the original owner (That's why Tesla produced a copy of Monroney Sticker with FSD.)
The most feasible rationale that I've read is:
How do you tell if a car has FSD vs EAP?
Tesla may want to unify its fleet options once it got a hold of a used Tesla in its possession. For example, all certain used cars would start to have plain Autopilot and no longer a mixture in the fleet with some have EAP and some have Plain Autopilot.