I'd see it more like the Writer's Guild, Director's Guild, or Screen Actor's Guild. They don't stop you from entering the field, or from doing smaller work, but

I'd see it more like the Writer's Guild, Director's Guild, or Screen Actor's Guild. They don't stop you from entering the field, or from doing smaller work, but once you start doing professional work, you would need to join the guild. (I don't know where that line would be drawn, and right now, it's not important to me.) The existence of such guilds does not mean that non-members can't buy paper, director's chairs, or cameras.