Take a Knee
The US Supreme Court has held that First Amendment rights may be waived upon clear and convincing evidence that the waiver is knowing, voluntary and intelligent. D.H. Overmyer Co. v. Frick Co., 405 U.S. 174, 185, 187, 92 S.Ct. 775, 782, 783, 31 L.Ed.2d 124 (1972). So, it is possible for me to enter into a confidentiality agreement where I agree not to disclose certain information (even though that agreement contradicts my right to free speech). The NFL players signed employment contracts with their teams that almost certainly include provisions where they agree to abide by the league’s rules. I read the NFL rules covering this issue and believe their protests violate the rules. It’s up to the teams or the league to enforce these contractual provisions and rules, but if the player wants to protest like this, it’s more a breach of contract issue rather than a matter of an exercise of constitutional rights. As noted above, I may voluntarily circumscribe my constitutional rights.
In full disclosure, I am a contracts law expert, but not a constitutional law expert. I also have not reviewed any specific NFL player contract and have only given a cursory read of the rules.
Finally, I support the players’ right to protest, but could not counsel someone to take an action that could give rise to civil liability for breach of contract.