When the Vegas Golden Knights score their second goal against the Edmonton Oilers—a nifty midair redirection of a shot from the blue line—the foghorn blasts and chants of “Go Knights go!” take a couple of minutes to die down. It’s a Thursday night in February at the new, 20,000-seat T-Mobile Arena on the southern end of the Las Vegas Strip. “They have cost us a lot of money,” says Joe Asher, who’s watching in a suite above center ice in blue jeans and a half-zip fleece. Asher, chief executive officer of William Hill US, oversees the largest collection of sportsbooks...