Yes, it’s not mandatory and yes you _could_ sit down and not sing, but the consequences are that your fellow spectators will be agitated. They may not show it in any way, or… …they might!
The interpretation of my intended message has transformed a bit beyond the original scope to include never standing, not even feeling social pressure of any kind, and a comparison to Colin Capernick who attends sporting events in an entirely different capacity than I do. Those points just sound a lot like trying to win an argument on technicality vs have a discussion about experiences.
All I'm saying is you can feel all the pressure in the world to sing a hymn or say a pledge at events, compromise half way by standing, and not sing any national hymn or pledge and suffer no compromises. It is absolutely not required.