> In an airplane you're in someone else's private space (ie the airline's) and everyone is not only confined with you in minimal comfort, they have no way to leave.
Its not private space. Its public because they sell tickets. Its like going to any other event, and I don’t think there’s a constitutional exception to free speech on airplanes where you can’t express your opinions.
It is perfectly legal to sell tickets to an event and require people going there not to express political opinions.
Flying on a plane is in no way similar to a public ticketed event. It takes about 30 second of logical thinking to realize that's not true.
I'm not sure if you are saying one can or can't express opinions on airliners.. but I do want to point out that the "contract of carriage" of most airlines is more restrictive than you might find for a ticketed event like a concert. You might want to read the one for United, just for fun (especially if you fly). https://www.united.com/en/us/fly/contract-of-carriage.html Rule 21, item H 16 even indicates that you can't smell bad. YMMV, but it is pretty far from a "public" space as I define one.