That's true, but unionized spots have such a strong individualistic component to the work. However, in industries such as warehouse workers, the employee ability is a lot more uniform, so I doubt there would be provisions for superstars to negotiate salaries.

It would more likely be something akin to a teacher union, with teaching being in between performance athletes and uniform workers. And even there I don't think individual teachers have much power to negotiate salary, but I could be wrong.