The difference is that the fans are paying for their tickets.
The players are being paid to play the game and put on a show.
If the fans decide they don't like what they are seeing, the league and the players are out of luck. If the league and the players don't like the fans they can either get new ones or go out of business.
It is the classic customer - supplier relationship.
Because you and some of your football playing peers are choosing to intentionally disrespect the anthem, flag, and country you da.