In the dusty town of Villa Tunari in Bolivia's tropical coca-growing region, farmers used to barricade their roads against U.S.-backed drug police sent to prevent their leafy crop from becoming cocaine. These days, the police are gone, the coca is plentiful and locals close off roads for multiday block parties—not rumbles with law enforcement.

"Today, we don't have these conflicts, not one death, not one wounded, not one jailed," said Leonilda Zurita, a longtime coca-grower leader who is now a Bolivian senator, a day after...