Flying on an airplane is a privilege, and airline employees' have a responsibility to ensure all passengers feel safe, secure and comfortable. As such, your rights pretty much end once you board the plane:
Federal Aviation Regulations state that “no person may assault, threaten, intimidate, or interfere with a crew member in the performance of the crew member’s duties aboard an aircraft being operated.” Generally, flight crews interpret that as giving them the right to remove any passenger for almost any reason.
That's ridiculous. How can airline employees penalize and harass passengers with impunity when there's no dress code or written standards made available to the public? If you're flying a domestic flight in America, you should be entitled to exercise your right to freedom of speech—even in the air. ✈️
Again, flying is a privilege–not a right. Airlines are well within their powers to force a passenger to remove clothing that could make employees or fellow passengers feel unsafe or anything that could cause a disturbance. They are literally following the law. You are not entitled to make other people feel unsafe.
Flying (or any manner of transportation) was not included in the Bill of Rights. It is not owed to any of us simply because we want or even depend on it. It is a privilege that must be earned and paid for.
But the rules for what clothing is deemed inappropriate is completely subjective and, let's be real, open to racial profiling. Would a middle-aged white guy wearing a "Black Panther" baseball cap from "Captain America: Civil War" be kicked off a flight? Unlikely. Airlines have a long history of racial profiling and ejecting passengers for dubious reasons. Leaving it up to the discretion of airline staff leaves such decisions open to blatant racism.