You make a good point. The operative word is “require” and the NFL has a loose philosophy about what is required of players. They follow the US Code of Etiquette which uses the word “should” rather than “must.” Legal experts argue the word “should” is not compelling. You might want to read a more in-depth piece on claireifyblog.com
BTW, I hear Kaepernick is now filing a lawsuit against the NFL.