The Bills don't have to submit a formal complaint or accusation to the NFL about Beckham if they believe he threw punches. And for purposes of discipline, it doesn't matter that Beckham wasn't called for a penalty during the game. The NFL reviews every play of every game for the purposes of potential discipline, and based on that review, NFL VP of Football Operations Merton Hanks can issue a fine or even a suspension. The league acknowledges that its officials can't catch everything that goes on, so a play does not have to have been flagged for a penalty during the game in order to result in discipline.
The good news for owners, the story pointed out, is a suspension for Beckham is highly unlikely unless the league uncovers something truly egregious. Even then, history indicates that they would start with a series of escalating fines and only resort to suspension once the player has been warned and fined several times. But on-field fighting is an officiating point of emphasis this year, and players have been warned that they can be ejected for doing it.