Sin City opens on April 1, I haven’t been as enthuiastic about a movie trailer since The Return of the King. And it’s got a cool backstory too. Director Robert Rodriguez dropped out of the Directors Guild of America (DGA) because union rules prevented him from naming graphic novelist Frank Miller as co-director.

DGA rules dictate that there be only one director assigned to direct a motion picture at any given time, although the guild occasionally grants a waiver to that policy.

On Thursday, a DGA spokesman said, “The guild regrets Mr. Rodriguez’s resignation, however, we stand firmly behind the principle of one director-one film.”

Rodriguez explains himself:

“I am making such a literal interpretation of his book that I’d have felt weird taking directing credit without him. It was easier for me to quietly resign before shooting because otherwise I’d have been forced to make compromises I was unwilling to make. Or set a precedent that might hurt the guild later on.”

Rodriguez often agrees with the spirit of DGA policies, but they “make it very hard to do something that is exciting and different, which is exactly how I sold this project from the beginning,” he added.

All this makes me wonder:

1. What’s in it for the DGA?

Like all unions, they want to hold down the number of workers in their industry. Rodriguez gets the microeconomics exactly wrong when he says:

“Someone in my position doesn’t need the protection of the guild as much as a newcomer who might get strong-armed by a film company.”

Ha! It’s the established directors who benefit from the unions’ effort to curtail new entry – and the newcomers who would just love to be “strong-armed” – which is also known as getting their BIG BREAK.

On further thought, though, it hardly seems like refusing to co-credit Frank Miller is going to put money in the pockets of established directors. It’s not like Miller is going to go solo. This seems more like bureaucratic inertia rather than a calculated shakedown to me. A little flexibility could have guarded the union’s mystique.

2. How does the DGA make its rules stick?

Apparently the DGA has failed to make its rules stick. The worst consequence, seemingly, is that Rodriguez won’t get to direct A Princess of Mars. But Sin City steamrolled ahead, and half of the stars of Hollywood appear in it. Not much of a secondary boycott. And it’s hard to believe that Rodriguez will have trouble getting choice projects after this.

Even more puzzling, Rodriguez has dropped out of the DGA before! How can a union restrict labor supply if it lets you jump in and out at your convenience?

For decades economists have wondered about unions for entertainers. How do they manage to control the supply of such an elastic product? Perhaps the Rodriguez case shows that they have a lot less control than we thought. No problem, no solution.