On April 15, 2011, thousands of Americans who played online poker woke to find three popular websites shut down by the FBI and millions of dollars in players’ accounts frozen. The online poker industry survived its “Black Friday,” as the day became known, as the Obama administration declared online gaming legal later that year in states that approved it. But the aftermath should serve as a looming worst-case scenario to the millions of Americans who play daily fantasy sports, legal experts say, as many of those players who had money saved in online poker accounts had to wait years before...