If you have the power to censor other people’s speech, special interests will try to co-opt that power for their own purposes. That’s a lesson the Motion Picture Association of America is learning this year. And it’s one that Internet intermediaries, and the special interests who want to regulate them, need to keep in mind.

MPAA, which represents six major movie studios, also runs the private entity that assigns movie ratings in the U.S. While it’s a voluntary system with no formal connection to government, MPAA’s “Classification and Ratings Administration” wields remarkable power. That’s because most movie theaters, along with retail giants like Wal-Mart and Target, won’t show or sell feature films that lack an MPAA rating. And a rating of “R” or “NC-17” can drastically limit the audiences who are allowed to view or buy a movie.

Power creates its own temptation. MPAA itself has been accused of rating independent films more harshly than those produced by MPAA’s own member studios. And this year, a class action lawsuit seeks to force MPAA to use its ratings system to eliminate tobacco imagery from children’s films. The lawsuit, Forsyth v. MPAA, claims that MPAA has a special legal duty to avoid harm to children, and because of that duty, MPAA should be required to give an “R” rating to every film that contains smoking or other tobacco use.

MPAA has responded by moving to dismiss the suit under California’s Anti-SLAPP law. The group argued that its movie ratings are a form of speech protected by the First Amendment. It denied having any legal duty to protect children from images of smoking. And MPAA argued—sensibly—that Mr. Forsyth’s claims are a slippery slope:

[Plaintiff] is trying to use the tort system to require [MPAA] to implement his policy goals. If Plaintiff’s claims were permitted to proceed, there would be no end to claims invoking [MPAA’s] purported duty to disregard its own opinions and instead to implement a given advocacy group’s preferred social policy in assigning ratings.

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Plaintiff’s theory . . . has no logical stopping point. The rule would require [MPAA] to give an R rating to movies that depict any conduct that advocacy groups think unhealthy—for example, movies that depict alcohol use, gambling, contact sports, bullying, consumption of soda or fatty foods, or high-speed driving.

MPAA is right. The First Amendment generally prohibits using legal processes to regulate the opinions expressed by others, no matter how noble the purpose. In fact, the slippery slope of censorship is one of the primary reasons why courts and legislatures can almost never regulate speech based on its content: if one form of “harmful” speech is banned or limited, it’s hard to avoid banning or limiting speech on every subject that some powerful interest finds harmful. We expect that MPAA will prevail in this lawsuit.

But there’s an irony to MPAA’s position in this lawsuit, because at the same time it fights to protect the ratings board against co-opting by special interests, the trade association is also trying to co-opt other powerful private gatekeepers of speech into advancing MPAA’s own special interest: copyright enforcement. Internet intermediaries like webhosts, domain name registrars, search engines, and third-party platforms are, like MPAA’s ratings board, private organizations that stand between speakers and their audiences. Their roles give them power to suppress speech, by making it harder for audiences to access, or even making entire sites disappear from the Internet.

Power, once again, creates temptation. This year, MPAA made agreements with two domain name registries, Donuts and Radix, which control new top-level Internet domains such as .movie, .online, and .site. Both registries agreed to receive accusations from MPAA that particular websites are engaged in copyright infringement, and to consider taking away those websites’ domain names. MPAA, along with other representatives of major entertainment companies, has also been pushing ICANN, the group that oversees the domain name system, to mandate this new copyright enforcement regime worldwide.

There are many problems with this initiative, which we’ll be exploring in the coming weeks. But one lesson that MPAA should have learned this year is that once one special interest obtains power to block the channels of communication, others will come knocking. Many powerful interests want the power to edit the Internet, from corporations and wealthy individuals who want to suppress criticism to repressive governments seeking to quash dissent. Some may even have widely supported (though controversial) social goals, like stopping “hate speech,” blasphemy, or pornography. Like the plaintiff in the Forsyth case, all of these folks want these private companies and systems “to perform a different function . . . one [they] make[] no claim to serve.” Just as MPAA is right to worry that the Forsyth case could open the door to more control of the ratings board by various special interests, new copyright enforcement systems will quickly become enforcement systems for all kinds of speech that a corporation or government declares to be dangerous.

This has already happened in the copyright realm: Major ISPs in the United Kingdom are now required to block their customers from reaching entire websites that are deemed to be copyright infringers, using a system that was originally set up to block child pornography.

But, you might say, copyright is a law, while preventing smoking is simply a policy goal. But just as MPAA has no legal duty to promote a zero-tolerance message about smoking, intermediaries have no legal duty to police the Internet for copyright infringement, or to prevent their users from infringing. And just as laws on “hate speech,” blasphemy, and sedition vary widely between countries, copyright is not the same everywhere. Depictions of tobacco use are themselves subject to strict “plain packaging” laws in some countries. The more Intermediaries on the global Internet are co-opted into regulating content, the more pressure they will face to apply the standards of the most censorious countries and organizations.

In the coming weeks, we’ll be exploring how speech on the Internet is being controlled by private agreements, and how Internet users can demand accountability and transparency in these Shadow Regulations. For now, even if the Forsyth case is quickly thrown out of court, it should serve as a cautionary tale: build a system that can regulate the speech of others, and the censors will beat a path to your door.