We fundamentally agree about who is in the right and the wrong.

The issue is timing and consequences. Freedom of speech doesn't mean freedom from consequences.

Consider:

Person makes loud free speech statement - which though perfectly legal - is contentious. Person B gets upset and uses their freedom of speech to shout back.

At the pub:

Everyone else is uncomfortable and just want to get on with their day. But they can leave. Usually they at least make space.

The pub can ask the talkers to leave - either one or both, as it is their private property. And if violence occurs Bouncer/Police drag away the violent person without much likelihood of harm to others.

On the plane:

Everyone else is uncomfortable and just want to get on with their day. But they cannot leave! They cannot make space! There may be elderly next to kids of varying ages. There is no bouncer, police is even worse for business and affects everyone on the plane just wanting to get to their destination. Tight constraints mean physical violence is likely to hurt someone unrelated to the conflict.

And so Captains clamp down on contentious statements ASAP to make sure it doesn't escalate that far. You are allowed to make contentious statements, and they are allowed to ask you to leave, because the plane is not public property.

Nobody wants to fly next to the loud nervous talker. Or the crying baby. But people understand these actions are not by choice and so there is tolerance.

Contentious/provocative statements though? That's a choice. Freedom of speech doesn't mean freedom from consequences.

It is better for everyone involved if the desire to spout views is simply delayed until the flight is over.