The NFL routinely bans misbehavior, even banning alterations from their dress code.
Secondly, the NFL does not want to be affiliated with a subversive group does it? People who mock their own citizenship? People who arguably revoked their citizenship?
Maybe these players should seek green cards to work in the US.
The NFL also banned players from politically supporting the police. So if one form of political speech is banned, why is that not sauce for the gander?