Everybody who is affiliated with the NFL, except for owners, are employees. Wherever I've worked there were rules about what you could say or clothes you could wear at my place of work.
Nobody was forced to work where I worked.
The same thing applies to these idiot football players. Any player performing political stunts or wearing clothes at their place of work not in accord with the league standards and practices should be fired. And so should their coaches who allow them to protest at their place of work.