We live in America. 
Employment does not come 
with a gag order. 

   For those who say the NFL players should not kneel during the flag ceremony because they are on someone else's dime, I would suggest that employment does not come with a buyout of our constitutional rights.
   If their time protesting took away from their time doing their job, that would be different. Employers can require you to do your job, as opposed to doing other things. But, the NFL players are not taking a single minute away from their jobs. 
  If the players were protesting against their employers, that, too, would be different. Employers should not be required to keep in their employ those who speak negatively of them. Nor should they be required to let those protesting against them do so on their property. But that falls way outside what the NFL players are doing.
   If you respect your players, give them the right to have their political opinions. Give them the right to petition for a redress of their grievances, even as the First Amendment gives them that right.