How can free software licenses be enforced through copyright law?

Free software licenses are enforced through copyright law by treating the license as a set of conditions on the use of protected work. Because the copyright owner grants permission to use, modify, and distribute the software only if specific terms are met, these licenses are legally binding.12

If a user violates these conditions—such as failing to keep source code open or neglecting to pass on the required license—the free license is effectively voided. Once the license disappears, any further use or distribution of the software constitutes copyright infringement, allowing the copyright holder to pursue legal action. This mechanism is central to copyleft, which ensures that software and its derivatives remain freely available to the community by mandating that successive users accept and transmit the same open-source terms. A significant legal milestone for this enforcement occurred in 2008, when a U.S. federal appeals court affirmed that such conditions are enforceable under copyright law.21