Architects are a well known example of artists that keep a say over their products after they have been produced and 'sold'. You can't just modify 'your' building: the original artist has to approve. Even if you're the only one living in it and you shield the building from public view.

When you buy a painting, the artist may forbid you from changing it.

Why do you believe that can't be the case for a movie? Just because it is technically possible to modify only copy does not make it obvious that you should be entitled to it.

Note that I'm not asserting that this actually is the case. I'm mostly asserting it's not obviously not the case.