The group wrote that lumping social media in with state actors, or issuing a broad ruling that permitted lower courts to do so, could "wreak unintended havoc on the rights of online speakers and the private platforms they use to disseminate their messages."
Breyer and Kavanaugh asked pointed questions during argument about which other companies might be affected if MNN is considered a public forum.
Kavanaugh asked Hughes, the attorney representing the producers, whether the state imposing regulations on companies would be enough to expose those companies to First Amendment liability, and noted the concerns that had been raised about companies such as Twitter and YouTube.
Hughes, who has argued that the facts at issue in the case would not apply to social media companies — or even all public-access television channels — responded that that situation would be different. He said there is a limitation on the government's ability to control speech on property it does not control.
"This is property that it does control," he said.
Breyer seemed skeptical, though, saying that "it's not so clearly different."
Breyer noted that the dividing line between MNN and other regulated companies could be difficult to determine, given the broad range of government entanglement in private U.S. firms.
Those comments formed the entirety of the questioning that could implicate social media, leaving the remainder of Monday's arguments to fact-specific issues related to how much control MNN had over the programming that runs on its platforms.
In particular, the justices appeared focused on whether MNN was permitted to curate its programming, with several of the court's liberals appearing intent on classifying the network as an agent of the state.
Roberts and Justice Samuel Alito, as well as Justice Elena Kagan, focused much of their probing on factual questions related to who owned the public-access channels.
The focus on ownership augurs well for large internet companies, which are not owned by the government, said Robbins, the former assistant U.S. attorney.
"That's a criterion that you would think cuts sharply against a finding of state action in the cases of social media enterprises," Robbins said. He said that the justices' focus on how much discretion MNN has over the content on its platforms also suggests that the social media companies are not in danger.
Justice Ruth Bader Ginsburg, who asked the first question on Monday, said that it appeared to her that MNN had no independent judgement, and therefore was "an administrator of a city policy," while Justice Sonia Sotomayor suggested that MNN was "the agent directly of the state."
The justices hinted toward a ruling that will be specific to this particular case.
At one point, Breyer interrupted Michael de Leeuw, the attorney for MNN, while the attorney was making a point about the court's previous rulings.
"I can read the cases," Breyer said. "What I can't do is figure out what the facts are."