The NFL only recently decided that players needed to go stand outside during the playing of the anthem as part of a paid "patriotism" push. Symbolic patriotism is not part of the job of a football player. Could the NFL, as an employer, theoretically compel standing as part of the "performance" aspect of football? Probably, but they are not likely to do so which still leaves open the question of what business does Trump have interfering with private employment?