Uh, yes, you absolutely DO own the car. Nobody can come over to your house and take it away from you. And if you sell a car with FSD, yes, it transfers to the new owner, and as a result, used cars with FSD sell for more than equivalent cars without it (though it depreciates just like the rest of the car).
What you're talking about however is how if you sell the car to Tesla, they'll strip out FSD before they resell the car, in hopes that the new owner will pay full price for it. Which is certainly their prerogative, as they now own the car after they pay you, and can do whatever they want with it.
Those statements feel a little mushy. Whose car is it? Whose FSD is it? If I buy a Tesla with FSD then I can sell the car, but I can't just sell the FSD. Tesla gets to sell the car with or without the FSD and gets to also sell the FSD to whomever they like. For you and I, FSD is part of the car. For Tesla it's like a wheel upgrade. For you and I, it is absolutely NOT like a wheel upgrade.
And I'm not saying that's wrong or bad. But it is kind of new and weird.