The MSM keeps talking about free speech rights of the players. Sure, as human beings we have the right to do anything we want so long as it does not harm someone else. (I know, not according to the US Government)
But the NFL can make any rules it wants regarding the national anthem. The right to free speech in the Bill of Rights is to codify that the federal government cannot restrict it. It does not apply to private organizations.
I assume those players can do what they want outside of the team regarding supporting ‘civil rights’ or whatever, as that would surely not violate any moral/criminal clause in their contract. But they do not have a “right” to do anything they want on the field without the possibility of being fined or fired.
disclaimer: JMO. I am not a lawyer.