Part of the problem is that nobody knows what they believe. You learn in school three facts: free speech is "liberal", free speech is "good", and the Democratic

Part of the problem is that nobody knows what they believe. You learn in school three facts: free speech is "liberal", free speech is "good", and the Democratic Party is "liberal" (Nate avoids this word for exactly that reason, and notes as much in TFA). So, a lot of people (subconsciously, without realizing it) make the following computations: "I am good." + "Free speech is good." = "I am pro-free-speech." "Democrats are liberal." + "Free speech is liberal." = "Democrats are pro-free-speech.", + "I am a Democrat." = "I am pro-free-speech." And then never actually check, internally, "do I support the idea of people I disagree with speaking their honest opinions out loud?" or whether they identify with that famous quote "I disapprove of what you say, but I will defend to the death your right to say it". There's various strategies of retreat, from misidentifying particular ideas as "intolerance" so one can misapply Popper's quote about the paradox of tolerance (see also "hate speech"), to being legalistic about it and denying that it's nothing without social norms too, to talking about deplatforming speakers as a form of free speech too (still forcibly silencing rather than counterarguing!), and you can see a dozen examples right on this page. All of them, I believe, have the same root: genuinely believing oneself to be pro-free-speech, caused by saturation of an American society that values being pro-free-speech, but without ever actually being convinced of the merits of free speech. And I think it has spread precisely because it's invisible unless poked; you don't need to convince someone of the merits of free speech if they already believe it, and so a lot less convincing has gone on than needs to.