If you look back at the thread, you'll understand. The Tesla Model X in question was purchased with FSD included, and after trade-in, it was removed as Tesla is reselling the used vehicle. Why?
It's been purchased, paid in full and is a part of the car. This is not a case of a monthly subscription or an aftermarket add-on that in non-compliant. Tesla shouldn't be able to re-sell FSD at the current price to make an extra profit. If you have a sunroof option in a car that you bought - does the manufacturer take it back if they decide to resell it as part of a trade-in?
If Tesla sets this precedent, then by that same rationale, then FSD should be transferable.