A private corporation that runs a public "forum" is not bound by the First Amendment, the Supreme Court ruled Monday morning.

The case, which nominally concerns a public access channel in New York, has attracted attention as a potential vector for regulation of social media firms facing charges of viewpoint bias.

The basic details of Manhattan Community Access Corp. v. Halleck are mundane. New York City designated Manhattan Neighborhood Network (MNN), a private nonprofit corporation, operator of a public access channel. Respondents DeeDee Halleck and Jesus Papoleto Melendez produced a film critical of MNN, which MNN agreed to air. The corporation subsequently removed their film and suspended the pair, claiming that they had made threats against MNN employees. Halleck and Melendez sued, claiming that MNN had violated their free speech rights under the First Amendment.

The case made its way to the Second Circuit Court of Appeals, which is where it got interesting. Normally, to assess a First Amendment claim, a court would first determine whether or not the alleged violator was a state actor. But in this case, taking its cues from an opinion of now-retired Justice Anthony Kennedy, the Second Circuit instead ruled that, while MNN was a private entity, its fulfillment of certain roles made it a "public forum," and therefore subject to the requirements of the First Amendment.

The question before the Supreme Court, then, was whether or not what the Second Circuit had done was legitimate. Or, as Halleck and Melendez put it in their initial filing, "whether the Second Circuit erred in rejecting this Court's state actor tests and instead creating a per se rule that private operators of public access channels are state actors subject to constitutional liability."

"Under what circumstances," they ask, "can a private entity (here a private operator of a public access channel) be deemed a state actor, subject to claims under the First Amendment?"

This question concerns more than just the operator of a Manhattan public access channel. Many major social media sites—Twitter, Facebook, YouTube, and so forth—operate as platforms for discussion, and thereby claim no legal responsibility for the content published on them. But if the First Amendment can be enforced against a private entity serving as a public forum, then these sites risk similar lawsuits.

This concern was enough to motivate amicus curiae briefs from both the Internet Association, a trade group representing a number of major tech firms, and the Electronic Frontier Foundation, the preeminent digital rights advocacy organization. The latter argued stridently against the idea that the mere operation of a public forum could qualify an otherwise private firm a state actor subject to the First Amendment.

"Certainly, the mere fact that something is either labeled a ‘public forum’ or operated by a private entity as a space generally open for communication by others does not automatically transform that private entity into a state actor," the EFF's brief reads in part. "Internet users' rights are best served by preserving the constitutional status quo, whereby private parties who operate private speech platforms have a First Amendment right to edit and curate their sites, and thus exclude whatever other private speakers or speech they choose."

A majority of the Court appeared to agree. Writing for his conservative colleagues, Justice Brett Kavanaugh concluded that MNN was categorically acting as a private actor, not a state actor. Because MNN does not cross that threshold into state action, he argued, whether or not it fulfills a role as a public forum is irrelevant to whether it is bound by the strictures of the First Amendment.

"[M]erely hosting speech by others is not a traditional, exclusive public function," Kavanaugh wrote, "and does not alone transform private entities into state actors subject to First Amendment constraints."

The dissenting opinion authored by Justice Sonia Sotomayor, and joined by the court's left, leaves more leeway for possible First Amendment obligations for public forums, insofar as they are acting on behalf of a government agency. Specifically, Sotomayor argued, "By accepting [an] agency relationship [with New York City], MNN stepped into the City's shoes and thus qualifies as a state actor, subject to the First Amendment like any other."

This leaves unclear what, exactly, constitutes a public forum subject to the First Amendment. Sotomayor notes that "this Court has not defined precisely what kind of governmental property interest (if any) is necessary for a public forum to exist." Her dissent, and the majority ruling as well, is silent on the question of public fora that, while private firms, rely on a government-created resource, i.e. the internet, in their model.

Still, the majority's ruling seems to preclude the application of the First Amendment to private actors like Twitter or Facebook. This is all the more significant because many prominent figures on the right—especially president Donald Trump—have invoked free speech norms to criticize perceived attacks by social media giants on conservatives. Today’s ruling means such an argument, at least in the courts, is unlikely to get very far.