The NFL has specific rules covering the entire gameday period barring the players from wearing any item or image that promotes a non-football personal message (commercial, political, etc.) unless approved in advance by the league.
If the NFL cared, they would extend that rule to include physical actions by the players that would have the same effect.
Kneeling for the Anthem or raising one's fist would (and should) be barred as they make a political statement that has nothing at all to do with the game.
If the NFL cared, they would also realize that such actions fall into the category of unsportsmanlike conduct, and should be added to the section defining such conduct.
Does the NFL care? We shall see.
I thought there were NFL rules and game rules that pertained to the players standing on field during the playing of the national anthem.