It shall be unlawful for any contractor receiving an operating-differential subsidy under subchapter VI of this chapter or for any charterer under subchapter VII of this chapter unjustly to discriminate in any manner so as to give preference directly or indirectly in respect to cargo in which such contractor or charterer has a direct or indirect ownership, or purchase or vending interest; and whosoever shall violate this provision shall be guilty of a misdemeanor.