Every farmer has a decision: Eat the grain, or sell the grain. They sell it if the money has more value to them than eating it. Unless they're indentured servants or slaves, they are doing this for their own benefit. If they'd starve as a result, they just wouldn't sell the crop.
If the local community has no economic integration and no means to afford the transportation costs associated with imports (because transportation has been optimized for a certain crop), then it's a problem.
Malnutrition has been an endemic problem in Bolivia since the Spaniards pillaged its mineral wealth. This does little to improve things.
