I think the players have the right to protest whenever they want. Its the 1st Amendment. But if they are violating some personal property law then ownership can then have them arrested for doing so. My guess is they never would. My guess is the team wouldn't release an integral pkayer that decides to do it because winning.
The NFL has the players wearing pink every year for breast cancer awareness. So as long as the owners approve the players promoting something they are behind its okay to do it on game day but the pkayers can't promote something they are behind during that time. Seems hypocritical to me.
Now let me say, I don't care if the players kneel in protest or not and I would never protest during the national anthem. But protests work when they can be seen by the most people so I understand why players would want to do it during games rather than write an article for the Player's Tribune or get involved in their local area to protest. Doing those things just aren't going to get to as many people as a nationally televised game.
I also can't help but wonder if Trump didn't make this a political issue to rile his base that it wouldn't be as big a deal.