If you try and interfere with someone who is peacefully sitting and eating their popcorn while the National Anthem is playing, YOU are the criminal. You can condemn them verbally on social media – that is your freedom of expression. But you have no right to touch them, or ask them to leave the theater.
Remember – if you respect your country, then you will respect/obey the laws. And the laws do NOT require someone to stand for the National Anthem.