You can’t miss the world’s largest sports-betting parlor, just past the bronze Elvis statue at the Westgate Las Vegas Resort and Casino. It’s a glitzy temple to the big business of laying odds on college and pro games, with 400 cushy leather seats and a massive wall of video screens. Westgate spent $18 million building it two years ago. And now that the U.S. Supreme Court has put an end to Nevada’s near-monopoly on sports wagering? To hear Westgate executives tell it, no problem. “We look at it as an opportunity,” said Jay Kornegay, vice president of race and sports...