Studios usually own the characters in their films and those character's images so for instance [blackout]Tarkin[/blackout] and his appearance as played by [blackout]Cushing[/blackout] belongs to LFL and Disney so they can use it how they want to. They can use it for merchandising and marketing. So why not use it in films as well? If Disney and LFL want [blackout]Tarkin[/blackout] in a film they can put him in a film and give him [blackout]Cushing's[/blackout] face and voice. It doesnt bother me and I enjoy this sort of thing when its done well and serves the story.
I'm pretty certain that isn't quite how it is. They own the characters and the likeness of the character, but that doesn't give them permission to just do as they please with the face of the actor, they don't own the actors face. It's one thing to use an image shot for the purposes of publicity, it's another thing to recreate somebody using CGI without them approving it. If Robert Downey Jr were to part ways with Marvel that doesn't give them licence to do as they please with his face just because he played Tony Stark.