Exclusive rights to something isn't exactly new. If you have a gym or Costco membership, you can't just let someone else use it because you won't be around for a month. If the farmer willingly entered into a contract with the supplier, the terms of the contract are enforceable.

Are you suggesting the government make rules on what kind of contracts parties can make in business? Would the government then oversee that? There are many, many contracts where there is an exclusive relationship described in a contract.