I think it's, for example, eminently possible to reconcile Twitter's right not to be obligated to carry "hate speech" (aside: please don't derail this discussion with an unnecessary diversion into whether that's a thing, or where its boundaries are; this is meta-level) with the President's not being able to block people.
If he's using the platform to conduct "official" business (which, IMO, he is), blocking is pretty obviously not kosher. That's an utterly different case than whether they should have to promulgate Joe Bigot's shitty opinions. We already have structurally similar legal doctrines in place in the case of libel against public figures, for example.
Looking for blanket solutions is the wrong idea, out the gate, however much more difficult and nuanced that will ultimately make handling the thorniness of the issue, taken large, and I think you actually already appreciate that, with your takes on the 4th and 5th Amendment questions.
Twitter is very much not obligated to pass along any random person's terrible views (and thanks for the disclaimer; no interest in seeing that argument again). If Trump decides to conduct official business there, they aren't quite facing compelled speech because they can (in theory) ban him; the outcome we'd hopefully get is that his official speech is restricted, but Twitter is free to host him or not.
I suppose at this point the only question is whether a government-proxy rationale applies to airports in this specific case, since I definitely agree that Kayak or Travelocity couldn't be required to accept these ads on their site.
If he's using the platform to conduct "official" business (which, IMO, he is), blocking is pretty obviously not kosher. That's an utterly different case than whether they should have to promulgate Joe Bigot's shitty opinions. We already have structurally similar legal doctrines in place in the case of libel against public figures, for example.
Looking for blanket solutions is the wrong idea, out the gate, however much more difficult and nuanced that will ultimately make handling the thorniness of the issue, taken large, and I think you actually already appreciate that, with your takes on the 4th and 5th Amendment questions.