Rather no star then for these.
But if such rating is added, this should not depend on the rarity of the car. Just if it is a background car having no real interaction but still a place in the movie, or a real background passing-by car.
You suggest that admins should do that for cars that they listed, I do not see what would be the other solution: it is like for the others ratings, it can not really be guessed correctly from the pictures.
And I disagree of allowing to list every car that passes the camera, it fills the database and disk, so it is not because we add something to hide these that they should be allowed.