The issue, though, is that her contract did not require her to participate in the sequel for a predetermined price which, as I understand it, is fairly standard language in the contract for a newcomer.
It works in favour of the studio, not the actor. Had Paramount offered more $$$$ than she received for ST II, or even the same amount(?), Alley would have been compelled to participate in ST III whether she liked it or not. But they offered her less, so she was free to walk. She wasn't expecting
to walk, but her agent's counter offer was ignored.