Challenges to the GNU General Public License alleging that it is unconstitutional are properly divided into two categories: those alleging that the bare license granted by the GPL is unconstitutional, and those alleging that enforcing the GPL under state contract law is unconstitutional. As a bare license, the GPL is in parallel with the copyright clause of Article I of the United States Constitution because it relies upon existing federal copyright law to ensure that the copyright holder's work is distributed according to the copyright holder's wishes. However, federal copyright law impliedly preempts enforcement of the GPL as a contract because the GPL fails the pass the "extra element" test. Accordingly, when it functions as a bare license, the GPL is a constitutional, conditional grant of rights from the copyright holder. However, attempts to enforce the GPL as a contract under state law would be null and void because the GPL is preempted by federal copyright law under Article VI of the United States Constitution.
Tennille M. Christensen,
The GNU General Public License: Constitutional Subversion,
33 Hastings Const. L.Q. 397
Available at: https://repository.uchastings.edu/hastings_constitutional_law_quaterly/vol33/iss4/1