Cowgal what you said is true, but consider that by purchasing a ticket a contract between the airline and the passenger was entered. They have an obligation to honor that contract and should not arbitrarily and capriously deny a passenger passage.
An even private industry is bound by certain laws. Would you support the airline not allowing to travel on their privately owned airline because of their race? Of course not, so why do you support them denying passage to someone because of thier religous beliefs or national origin when those passengers have been cleared and do not pose a security threat?
Suppose you were denied passage on an international flight for your dream vacation or an important business meeting and were denied passage because your shirt had a slogan on it in English? I'm sure you'd understand and leave the plane without a fight - right?
I saw a guy with the t-shirt stating Freedom isn't free. Some might feel that is offensive - should he be kicked off a plane because a Muslim finds it offensive?
Unless you are willing to accept the same rules for yourself, then you need to defend the rights of others.