The NFL chose not to enforce it's rule, but the players would be in breach of contract if it did. It is the League that is trying to have its cake & eat it too. They want to have the rule AND appear to have an enforcement mechanism. But does anybody seriously think Goodell is going to collect a fine from Robert Kraft or Jeffrie Lurie?
It’s a contractual one.
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Notice that the NBA hasn’t had this problem. They have a rule in the league
rules that states the players must stand on the foul line or the line in front of
their bench during the National Anthem.
This is the NBA rules which are a part of each players contract they sign
to play in the NBA.