Public accommodations may not be denied based on race, gender, national origin, etc.
Political views are not one of those protected categories, nor should they be.
Also, there are right and wrong ways to say things. We teach this children, but I will teach it to you, because maybe you didn't learn it when you were a child.
If you like Trump, just say, "I like Trump". It's a positive message, it's only about you, few people will have a problem with it. Don't say, "F### Joe Biden." That's a negative message, and a vulgar message. It's pretty close to fighting words. I'm sorry that sounds didactic and condescending but apparently that's what we need around here.
I mean, say it if you want to say it, but don't be surprised if people react the way this flight attendant did. You have the freedom of speech but not the freedom from the consequences of speech. If you put fighting words on your apparel, don't be surprised if people want to fight you.

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