Extra context from LWN:

Back in November, Vizio had asked to move the case to Federal Court, because the GPL is only a copyright license (which is a dispute handled at the Federal level) and not a contract (that could be adjudicated in state court). Friday’s ruling disagreed with that premise. […] “The ruling is a watershed moment in the history of copyleft licensing. This ruling shows that the GPL agreements function both as copyright licenses and as contractual agreements.”