I think this means that if the merchant does not have a written return policy MasterCard assumes the de facto policy is "no returns". This seems sensible because merchants generally have no obligation to accept returns, and it is the customer's responsibility to check before buying. However, the situation changes if the product is defective, and the transaction could then be formally disputed if it meets the criteria in the Fair Credit Billing Act, and this requires adhering to a very specific written procedure. Telephone calls will not preserve your rights.