The SAG is a labor union. “Guild” is just part of the name.
That said people just have problems understanding how unions work. It's a collective voice. How that voice is used is up to the union members. If you want that to include gate keeping and seniority protections that's up to the members. If you want it to be a political lobby, same thing.
I've never been in (or had the opportunity to be in) a union, but it seems like you could apply this exact same argument to the US congress. Congress is a collective voice. How that voice is used is up to the voters. Of course, it's far from that simple in the real world.
I also don’t thing developers need a traditional labor union. We probably need baseline guidelines where non-compliant companies would be seen as ‘very lame, avoid’. Similar to a coding standard.