This appeared in the Los Angeles Times:

Pressured by governments around the world, four companies operating some of the world's most popular Internet sites and services - Facebook, Twitter, Google's YouTube and Microsoft - announced this week a joint effort to censor "violent terrorist imagery or terrorist recruitment videos or images." It's an effort to fight a bad use of technology with more technology, in the hope of curtailing the use of social media by Islamic State and other terrorist organizations to recruit followers and promote their murderous agendas.

According to a blog post by Google, whenever one of the four companies deletes a terrorism-related image or video, it will have the option of submitting the file's unique identifier to a shared database. The other companies will then review the file to see whether it violates their terms of service and, if so, they can use its unique identifier to delete it from their pages.

The database is similar to the one the National Center for Missing and Exploited Children uses to catalogue and remove child pornography. And the goal of the companies' effort is similar as well: to push jihadist material out of the most widely used spaces online, making it harder for Islamic State and its ilk to radicalize people remotely.

The announcement is both welcome and worrisome. We may all agree that jihadists shouldn't be allowed to use these platforms to distribute beheading videos, but that's just a fraction of the material used to recruit and radicalize. Should speeches by Islamic State leaders and sermons by extremist clerics be censored too? What about news photographs of, say, victims of Israeli or American airstrikes, or photos of detainees tortured in Abu Ghraib?

Clearly, the line between what is and isn't acceptable will be hard to draw. Yet the slope the companies have started down is slippery. Governments will surely push to extend the blocking effort to more online sites and services, such as Telegram (a messaging app that's popular with extremists) and Google's search engine. And if the effort to interdict jihadist propaganda is successful, what other kinds of content will governments want these platforms to exclude? Hate speech? Indecent material? Fake news? Cartoons that ridicule the government - a form of expression that's a crime in some parts of the world? The precedent being set here is that a handful of powerful private companies could take the place of courts and juries in setting limits on speech that, because of these companies' collective dominance online, would apply broadly across the Internet.