A party can become a joint author, and then they can change the license on the entire work. The part "joint copyright holders can distribute the entire work under any conditions they feel like, even without agreement from any other joint copyright owner" is well established. You may not believe me, but it is the law (I already linked to the University of California, and Eben Motown's statement that to enforce a licence you need the cooperation of each copyright holder, acknowledging that a single dissenting copyright holder can kill an enforcement action; but here's a link to Stanford: http://fairuse.stanford.edu/overview/faqs/copyright-ownershi...).

The FSF avoids this problem by requiring copyright assignment. To be honest, I'm not sure that's enough, but I think they're hoping for a sympathetic judge. Most projects have never looked at what the law actually says regarding joint work, and think it works the way they would write the law. I know what the outcome of that will be.

I don't think that your claim has been tested in court when it comes to code. This might be true for some kind of music or something, but I find it hard to believe that a court would agree that two people who have never met and have only communicated over the internet would have formed an implict contract allowing relicensing of a work several orders of magnitude larger than the contribution of the relicenser.

It's like claiming that someone shouting outside of a recording studio has the right to relicense the music created inside the recording studio. While it might be true, I'd want to see an actual legal case before claiming things that are that outlandish.

Not to mention that the GPL has been tested in court and has held. So there's that.

It's not a question about contract. It's about how joint copyright works.

I know what the arguments would be, but I'm not sure how the court would rule. The hypothetical depends on legally making a fork, and the GPL (as an example) only gives permission to fork if you agree that any distribution will be under the terms of the GPL. If you violate those terms, you've committed copyright infringement. But can a joint copyright owner be guilty of infringing his own work?

I remember Eric Raymond discussing a commercial copyright infringement case where a contractor was hired to update a program, and started distributing the updated program without permission. This was his argument.

However, he hadn't made significant changes to the program. Using your words, the program was several orders of magnitude larger than what the contractor added. The court got creative, and declared the contractor only had a collective copyright -- like somebody who has copyright on a single article in an encyclopedia -- and not a joint copyright -- even though his changes were meant to be combined with the original work to create a single work and not a collection of works. A collective copyright owner only has copyright in his contribution.

You are right that courts would be reluctant to allow the tail to wag the dog, but there is some threshold where the contributor has made enough changes that he has joint copyright, and at that point he can do what the law says a joint copyright holder can do.

"Eben Motown" was Android's autocorrect's fault. I know it's Eben Moglen.