Notice a very important part of that rule: should.
Compare that to the NBA rule: 2) Players, coaches, and trainers are to stand and line up in a dignified posture along the sidelines or on the foul line during the playing of the National Anthem.
Should is a suggestion. Suggestions can be ignored. And kneelers are on the field. And really in the rulebook all your looking at is yardage, possibly ejections. Firing people needs to be in compliance with the CBA.
Under the salary cap, if an NFL team fired 10 or 12 players, they literally would not be able to field a competitive team for at least two years.