But then in February on the Sam Harris podcast “Making Sense,” Mr. Dorsey veered into an even more convoluted explanation: “Ultimately, I don’t think we can be this neutral, passive platform anymore because of the threats of violence, because of doxxing, because of troll armies intending to silence someone, especially more marginalized members of society,” he said. “We have to take on an approach of impartiality — meaning that we need very crisp and clear rules, we need case studies and case law for how we take action on those rules, and any evolutions of that we’re transparent and upfront about.” He continued: “I do believe that a lot of people come to Twitter with the expectation of a public square, and freedom of expression is certainly one of those expectations. But what we’re seeing is people weaponize that to shut others’ right to that down. And that is what we’re trying to protect, ultimately.”

It’s not hard to imagine why anyone, especially a twitchy tapper like President Trump, would respond with: Say what? The strategy over at Facebook — which also long defended the presence of its most noxious users before finally barring several last week — is similarly as clear as mud.

But let me break it down for those who have gotten used to the chaos: Social media companies are private entities that can moderate any of the content that floods their platforms. They can kick off users who violate whatever policies they have in place, change those policies anytime they like and be wildly inconsistent in how they enforce them.

That’s entirely legal under current law, which was girded by the 2010 Supreme Court decision in Citizens United, which said that corporations have free-speech rights, including the right not to speak. That means they cannot be forced to host dreck if they don’t want to.

That, of course, is scary to the slippery-slope crowd, who worry that a small coterie of mostly male, mostly white, mostly obscenely wealthy people are making such enormous publishing decisions for everyone. As Bret Stephens, a Times columnist, asked in the wake of the Facebook purge: “Do you trust Mark Zuckerberg and the other young lords of Silicon Valley to be good stewards of the world’s digital speech?”

Mr. Stephens does not, and it’s clear President Trump doesn’t either.

Some of those who have been kicked off social media platforms for their extremist or violent views have sued, pointing to a few previous Supreme Court and state decisions that have extended public speech into private analog spaces. Others are positing that services like Facebook, Twitter and YouTube should be regulated like public utilities.

What’s obvious is that the rules are not clear in a world in which the idea of the public square has been turned on its head. It is a truly challenging problem for democracy in the United States and for its increasingly voluble citizens who are now experiencing limits to what they can say.