> they negotiate directly using the power of being able to shut down production This is true, but they would slowly lose their leverage if they didn't constric

> they negotiate directly using the power of being able to shut down production This is true, but they would slowly lose their leverage if they didn't constrict the labor supply. In order to join SAG you have meet certain requirements (e.g. speak a certain number of lines). The vast majority of actors in a union production have to SAG members, so this effectively acts as a rate limit to the number of new members. The same goes for other powerful unions. Ford would love to build a new factory that employs non-union workers, but they aren't allowed to.