Isn't that slightly different and more because the film industry is unusual in that it has adapted around the unionised environment? Stars would have joined the union early in their career for the protection, but late on its minimal expense for sake of keeping the cogs of production turning. I'm no expert, but I imagine there are various on-set insurances which require actors to be a member of union, etc. It's not quite the same as a VP of a tech company joining a union.