WGA jurisdiction to determine the writers of movie and TV scripts

If the Writers Guild had not been created by writers themselves, movie and TV companies would have created it - just to have it determine who gets credit, when scripts are the creation of more than a single person. What gives the WGA the power to perform this critical and sometimes contentious task? That's the question addressed by UC Irvine law professor Catherine Fisk in The Jurisdiction of the Writers Guild to Determine Authorship of Movies and Television Programs. From the intro:
Ever anxious about their status as the authors of a film in a world in which directors and actors are far more likely to be seen by the public and by critics as the authors of a film or TV show, writers use their jurisdiction over writing credit to protect their status as authors. While jurisdiction is always the device by which law sanctions the exercise of power, the jurisdiction of the Writers Guild reflects and creates a particular form of power in Hollywood: the power to define individual authorship of a collective creation, which in turn creates the power to be paid to tell stories.