If two people work on code together, what is to stop one of them from making it open source? Is the sole impediment the risk of legal action by the other?
1 Answer
Yes. But there might be more.
If you release material under copyright with authorization, the only recourse is the (other) copyright holders dragging you to court and suing you on grounds for copyright (and maybe trade secret) violation for damages.
Depending on jurisdiction and the extend and effect of your infringement you might also be subject to prosecution by the state itself.
Whether that constitutes "only" depends - just think of the file sharing lawsuits. It's exactly the same thing.
The "more part": As you didn't have the right to distribute the material under the license, the possible recipients might also have a case to sue you for damages - if they relied on the license and thus might not be able to continue to use the software which they started to build a business on. After all they never received a valid license.