Have said it before and will say it again.  When you are on your own time you can protest or tweet or do whatever.  That is protected by the 1st amendment.  But when you go to the office, you check your politics at the door.  For NFL players, the playing field on Sunday afternoons is the office.  It is their place of business.  What the players are doing is no different than a GM worker distributing political leaflets in the company cafeteria.  It is offside.  And nobody would object if GM insisted they stopped.  The NFL is within their rights to demand that players uphold reasonable standards while they are at work.  That includes standing for the national anthem and making political statements on their own time.