No, you’re completely misunderstanding the rule. Because the film industry works on a freelance basis, the union only has bargaining power if it doesn’t exist alongside a non-unionized body of workers who are willing to work for cheaper. By requiring that union members only work unionized jobs, they ensure that no non-union production can ever benefit from any non-union labor. This pushes productions to negotiate terms with the union in order to get talent, and that in turn helps union members get jobs.

It’s not about whether the union likes you or thinks you deserve to work. It’s about whether the production is willing to play by union rules.