The theory, as it is usually laid out, is that intellectual institutions must protect free speech and robust debate from censorship and political pressure. Hypothetically, you could do this - while still enforcing all kinds of norms and civility - by laying out objective and unambiguous discourse rules about etiquette and argumentation. And occasionally, some institutions have tried to do just that. But the danger - which has played out time and time again historically - is that powerful people will try to game these rules in order to exercise de facto censorship. There are all kinds of obvious ways that this can happen: you can write rules that ban as "obscene", "blasphemous" or "unprofessional" or "rude" some things but not others, you can selectively enforce the rules, and so on.
Instead, the solution that liberal intellectual life has consistently returned to is to avoid any policing of speech. While this does leave open the danger of uncivil and unconstructive discourse, liberalism has always viewed that as a preferable alternative to placing instruments of censorship in the hands of power. This, by the way, is precisely the rationale underlying the First Amendment's absolute ban of speech restrictions. Literally centuries of liberal thought and jurisprudence have recognized that when powerful institutions - including the government - are given any kind of pretext to censor, they will be tempted or pressured to use it.
And that's also why we have tenure. Liberalism insists that, in addition to the government, intellectual institutions also have a responsibility to protect free speech. Tenure disarms the powerful of the means to censor by disqualifying speech rules as grounds for dismissal. Anyone with any minimal experience in academia - where the discourse is frankly far more combative and uncivil than anything anyone ever sees online - knows why this is necessary.
The subtext of all of this, of course, is the chilling series of tweets we saw last night from Demos scolding one of its own writers, Matt Bruenig, on behalf of two of the more powerful figures in elite liberal media and policy. Gasp and wring your hands all you like at personal attacks like "troll", "go to hell", "dudebros", comments about what "takes a lot of intellect", and so on - oh wait, those were all written and disseminated by the people in that argument who were not Bruenig! - but what is clear here is that Demos seems to be deciding that their own writer does not deserve the protection of liberal forbearance so vital to intellectual discourse and free speech.
This is a curious decision from an organization whose motto is "An Equal Say And An Equal Chance For All." It will be interesting to discover what they think those words mean.