I think "integrity" here means "honesty." In my town in the U.S. we have a "farmer's market" every Wednesday on the town common. Farmers set up booths and sell fruits and vegetables.
I can imagine a town committee that runs a farmers market saying:
"The purpose of the farmer's market is to provide an alternative to the supermarket and let people buy local produce. We have always allowed farmers to sell produce that wasn't grown on their farm, but last year there was a case of someone selling produce that wasn't even grown in the United States. This year we have added a rule that everything sold at the farmer's market must have been grown in the state of Massachusetts. We must protect the integrity of local produce."