Actually, when they are on the field they are at work, so their employer has the legal right to set the rules. They certainly are allowed to do whatever they want with the anthem when they are not at work. It's up to the NFL and owners of teams to do something about the protests if they so desire. If say Jerry Jones starts fining his players who kneel, he has every right to do so, the players can go find another job if they don't like the rules.