The hand-printed signs, in neat block letters, appeared in the Tulum marketplace the morning after two tourists were shot dead and three others wounded at a roadside eatery in the bohemian Mexican resort town. “Attention merchants of Tulum … this was a warning,” said the sign, which went on to threaten “managers and owners” of bars and restaurants on the “Mini Quinta” tourist zone. That’s where the foreigners, visiting the Malquerida Bar last month, had the bad luck of getting caught in cartel crossfire. The signs were photographed by a local citizens’ advocacy group, which posted them to social media....