The National Labor Relations Act provides that workers can form a union. The NLRA has been around since before World War II. Yet, we often forget that a major portion of the Act protects preliminary activity. Workers can discuss terms and conditions of their job. That sort of discussion can lead to the formation of a union. So, when Jerry Jones says he will not let a player play if he does not stand for the national anthem, he may be transgressing on the NLRA. Local 100 of the United Labor Unions has filed a claim with the National Labor Relations Board claiming his new policy violates the NLRA. See Bleacher report.
The NFL is a bit different from other industries. It is essentially entertainment. One might expect that an entertainment industry might have greater latitude in restricting employee conduct. But, that is just not likely. After all a strike itself would be very upsetting to fans and would certainly interfere with the business of the NFL. Yet, there is no doubt that a player has the right to strike. Flowing from that right to strike is the right to discuss terms and conditions of employment. That is what forming unions is all about. I am a veteran. I would always stand for the anthem. But, the NLRA is the NLRA. The law is the law. Respect for our laws is a duty for everyone, even NFL team owners.