The problem with free speech in the US is that it's only a protection against the government.
For reasons I've never really understood, we treat the coercive power of private non-person entities differently.
So, a woman gets kicked off her flight because fellow passengers don't like the political speech on her t-shirt. She's allowed to get on the plane, allowed to fly part of the way, but when it starts to bother the other people on the plane too much, she is asked to leave at the next opportunity.
The airline, Southwest, probably didn't do anything legally wrong.
In a country where we have no meaningful public transportation, it should be worrisome that people are denied transportation because of their political expression. Transportation and mobility are fundamental to the exercise of other rights.
It should also be worrisome that we are in fact only pretending not to have any public transportation. The airlines receive substantial public funds and support. Yet, they retain a private character, so they don't have to worry about little things like kicking a woman off her flight because she is wearing a t-shirt that insults the government in power.
Oh, and by the way. What kind of plane are these people riding? In what possible world could someone's t-shirt disturb another passenger on the plane once everyone is seated? You've really got to go out of your way to look at someone's shirt, and you'll probably get a neck cramp in the process. They must have been seated in those rare bulkhead seats where you face each other. How silly.