Fair point flotation device. it might be advisable for the airlines to state up front that they reserve the right to refuse service to anyone who displays inappropriate attire but there is no legal reason why they are required to do that. Secondly, I think this argument is more powerful if the passengers had no means of changing their attire once they found out. But they did, of course, by merely turning it inside out.
The reason I put up this topic is because several members of this board have, in the past, taken an absolutist view of free speech issues. Not only must the government not inhibit the exercise of speech (which is all the first amendment requires) but society is harmed when there is any repercussions to speech, even if it's a private company making reasoned judgements about what is and is not appropriate. I put this down because I wanted to see how this example would be fit into that framework.