The owners of the NFL’s Miami Dolphins, the MLB’s Chicago Cubs, the NBA’s Philadelphia 76ers, and the NHL’s New Jersey Devils have invested in a sports betting network and gambling startup. Action Network is another move by league owners to capitalize on the growing trend of legal sports betting. NFL owners long opposed the spread of legal sports gambling, citing fears that it would jeopardize the integrity of its $15 billion enterprise. Following a Supreme Court decision last year to strike down the federal ban, the league’s stance has softened. Over the last 12 months, the NFL has signed its…