Perhaps even more ominously, Twitter emerged during the 2016 presidential election as the pre-eminent means by which pro-Trump troll accounts—many of them automated, located outside the United States, often in Russia—disseminated false news and attempted to drive opponents off social media altogether.

Twitter has responded by periodic crackdowns against people known or suspected of organizing abuse. Breitbart.com’s Milo Yiannopoulos had his verification status stripped, and ultimately his account suspended, for his role fomenting the campaign against Leslie Jones.

In the case of Richard Spencer, however, there is no evidence of harassment or incitement to harass. The same can be said of most (although not all) of the other accounts suspended on November 15. These suspensions seem motivated entirely by viewpoint, not by behavior.

Let’s pause here for the necessary caveats: Twitter is a private actor; it has no First Amendment obligations to anybody. It is a for-profit entity, seeking to maximize the value of its service. It can turn away anyone it likes, subject only to non-discrimination laws—and personal belief is not a forbidden ground of discrimination. Twitter is acting wholly within its rights.

I’ll add a second group of caveats; these maybe a little more controversial than the first. Social-media platforms are not common carriers. They are entitled to turn away customers who behave in ways inconsistent with the platform’s identity and purpose. Americans have wide rights to post and view pornography—but there are strong business reasons why Instagram, for example, insists that Americans exercise that right somewhere else. If Twitter decides, “This is not the place for political discussion,” that’s Twitter’s prerogative.

But, of course, that’s not what Twitter has done. Politics remains welcome at Twitter, as its most famous user, the president-elect, can attest. What Twitter is saying is that some and only some speech will be policed, by standards that can only be guessed at in advance.

That’s socially undesirable for a lot of reasons, but consider just this one: It’s precisely the perception of arbitrary and one-sided speech policing that drives so many young men toward radical, illiberal politics. On campus especially, but also in the corporate world—and now on social media—they perceive that wild and wacky things can be said by some people, but not by others. By useful comparison: On the very same day that Twitter suspended the accounts of some alt-right users, DePaul University forbade a scheduled appearance by the broadcaster and writer Ben Shapiro. Shapiro is not an alt-rightist; in fact, the Anti-Defamation League reported last month that Shapiro is Twitter's single most frequently targeted victim of anti-Semitic abuse by alt-rightists. But Shapiro is a scathing polemicist and provocateur—an alumnus of the same Bannon-Breitbart empire that incubated Milo Yiannopoulos—and DePaul expressed worry that his appearance on campus might provoke violence.