If the collective Bargaining Agreement does not cover standing for the Anthem, what would happen if an owner cut a player for It? My guess, he owes the player salary AND takes the hit on the salary cap. Cutting a higher end player would doom a team for a couple years as they unravel the cap issues. Given most players are black, it probably hurts them with free agents.
The NFL made this bed, they have tried so hard to fit alongside "God and country". It was a marketing move that got players out on the field for the Anthem, they returned to the locker rooms before 9/11.
I will admit I think baseball is as bad marketing patriotism. I like patriotism, I do not like marketing it for profit. Adding God Bless America to the 7th inning stretch was a crass ploy (after the first, I will buy that original).