Anyone has the right to engage in political protest on their own time.  But on the job, you are supposed to be focused on your work, not on your personal politics.  On Sunday afternoons, NFL players are on the job.  They go to their place of business - the football stadium - put on their uniform - the team colours - and do what they are paid to do - play football.  The anthem is part of the game. They are on the clock when they are on the field.  They should leave their politics at home. Tweet all night if you want to, but show up 100% focused on the game. 

How would GM react if a bunch of line workers showed up at the plant for their shifts and wanted to carry anti or pro Trump placards in the lunch room.   Same thing.  The players just get paid more.