An association organized under this subchapter and complying with the terms hereof shall not be deemed to be a conspiracy or a combination in restraint of trade or an illegal monopoly or an attempt to lessen competition, or to fix prices arbitrarily. The marketing contracts and agreements between the association and its members and any agreements authorized in this subchapter shall not be deemed illegal as such or in unlawful restraint of trade or as a part of a conspiracy or combination to accomplish an improper or illegal purpose.