No labor union, or person acting in its behalf, shall require any person, as a condition of securing or continuing employment, to pay any fee or assessment other than such initiation fees, dues and assessments as are, by the constitution and by-laws of such union, chargeable upon members thereof. Any union or person violating any provision of this section shall be punished by a fine of not less than five hundred dollars.
Mass. Gen. Laws ch. 149, § 150B