Why did a U.S. appeals court rule that free open-source software licenses can be enforced under copyright law?

In 2008, a U.S. federal appeals court ruled that free licenses are enforceable under copyright law because they establish specific conditions for the use of copyrighted material.1

While these licenses are often viewed as contracts, the court determined that they function as conditions on the use of the work. Consequently, if a user violates these terms, the license is effectively voided. This transition means that the unauthorized use of the software constitutes copyright infringement rather than a mere breach of contract. This legal precedent significantly strengthened the protections available to creators of open-source software, ensuring they maintain control over how their code is modified and redistributed.1