Let's not lose site of one thing, "freedom of speech" !!
Yes, I agree it's disrespectful to stay seated, but it's not against the law. We all have a right to our opinion and so does he whether we like it or not. He will pay the price for his decisions.
I always stand for the National Anthem and The Pledge. A lot of people stand for God Bless The USA. Sometimes I stand, sometimes I don't.
I also see people standing for Neil Diamond's Coming To America. I only do if I'm dancing.