The NFL owners would be free to fire the players, however the NFL owners are supporting the players. It is clearly the right of the owners to express their political opinions. Note, that is why the owners could fire Alejandro Villanueva for not joining the owner-endorsed protests. Earlier protests were tolerated by the owners, the one’s this week were endorsed by (and in come cases, directed) by the owners.
And of course it is perfectly legal for fans to refuse to buy tickets or watch the games on television for the same reason – or to buy an Alejandro Villanueva jersey . The unknown in the equation is to what extent football fans are “blue” or “red”, and how that affects the teams’ bottom lines.
I myself prefer baseball, a game involving more intellects, more skill and less brute force — and in which almost half the players on American teams are from foreign countries (and with less overrepresentation of African Americans, which is also a factor in the protests), but if my favorite baseball team started protesting against my country, I would stop going to their games.