Cameron Heyward, the veteran Pittsburgh Steelers defensive lineman, remembers that when he was selected in the first round of the 2011 N.F.L. draft, the most popular betting event was the N.C.A.A. Division I men’s basketball tournament, which attracts millions of casual fans in the United States through bracket pools among friends and co-workers.

The league had policies back then to restrict gambling among players and others in the sport, saying that it would risk damaging the integrity of the game. But, Heyward believed at the time, the guidance was less of a big deal because betting was not as widely available.