At least one positive thing emerged from the Tazreen Fashions tragedy in Bangladesh. After a devastating fire claimed the lives of at least 112 garment workers in November, Walmart is tightening its grip on its supply chain. In a 10-page letter sent Tuesday to suppliers of Walmart and Sam’s Club stores in the United States, Canada, and the United Kingdom, the world’s No. 1 retailer says it will adopt a “zero tolerance” policy on subcontracting without authorization, effective March 1. The stricter regulation will replace Walmart’s previous “three strikes” rule.