Although Jacobsen v. Katzer (2008) seems to give teeth even without dual licensing. Katzer argued that the open-source conditions are "non-economic", and theref

Although Jacobsen v. Katzer (2008) seems to give teeth even without dual licensing. Katzer argued that the open-source conditions are "non-economic", and therefore not enforcable as a copyright infringement case, but the Court of Appeals, Federal Circuit wrote > Traditionally, copyright owners sold their copyrighted material in exchange for money. The lack of money changing hands in open source licensing should not be presumed to mean that there is no economic consideration, however. There are substantial benefits, including economic benefits, to the creation and distribution of copyrighted works under public licenses that range far beyond traditional license royalties. For example, program creators may generate market share for their programs by providing certain components free of charge. Similarly, a programmer or company may increase its national or international reputation by incubating open source projects. Improvement to a product can come rapidly and free of charge from an expert not even known to the copyright holder. The Eleventh Circuit has recognized the economic motives inherent in public licenses, even where profit is not immediate. [---] The clear language of the Artistic License creates conditions to protect the economic rights at issue in the granting of a public license. These conditions govern the rights to modify and distribute the computer programs and files included in the downloadable software package. The attribution and modification transparency requirements directly serve to drive traffic to the open source incubation page and to inform downstream users of the project, which is a significant economic goal of the copyright holder that the law will enforce. Through this controlled spread of information, the copyright holder gains creative collaborators to the open source project; by requiring that changes made by downstream users be visible to the copyright holder and others, the copyright holder learns about the uses for his software and gains others' knowledge that can be used to advance future software releases. The parties later settled, so we don't know what damages could have been awarded, but this affirms the basic principle of the GPL.