To all those who think the Constitution protects the rights of NFL players to protest during the national anthem, you’re wrong. The First Amendment does protect speech, including political protest. When a player of his own free will agrees to be employed by the privately owned NFL, however, that player agrees to abide by the rules established by that private entity.
Can an office worker kneel in protest at the beginning of every staff meeting? Can a waitress kneel in protest before taking every order? Can a casino dealer kneel in protest before every shuffle? Absolutely. But none of those people would have those jobs for very long.
Players work for the NFL. You want to protest? Do it on your own time. You don’t obey the company rules, you don’t keep your job.