Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
That's the first amendment to our constitution. Congress shall make no law... abridging the freedom of speech... One of the most important founding principles of this problematic country, is that we are free to speak our minds. It is, in my opinion, the principle that defines us best, and the only one that can eventually lead us to a country that is truly just. And, the rub, of course, is when people speak in ways that we happen not to like. The statement, "I hate what you are saying, but I will defend to my death your right to say it." is a reflection of the complexity of this particular founding principle.
The national anthem is a cultural instrument. There is no law, nor could there be, that forces anyone to do anything in particular during the anthem. You could sing "God Save the Queen" at the top of your voice if you wanted to. You could dance, you could stand on your head. And, you can sit. That's called freedom of speech.
Thoughtless patriotism leads to tyranny. If we don't want tyranny (few people actually do) we need thoughtful patriotism. And thoughtful patriotism says that when someone sits during the national anthem, they are expressing their freedom of speech. Yay!