It's in the IBT Constitution, and in the Membership Oath of your Local Bylaws as well. In theory you can be brought up on charges for violating the Oath.
"Article II --- JURISDICTION, MEMBERSHIP AND ELIGIBILITY TO OFFICE
Membership
Section 2(a). Any person shall be eligible to membership in this organization upon compliance with the requirements of this Constitution and the rulings of the General Executive Board. Each person upon becoming a member thereby pledges his honor; to faithfully observe the Constitution and laws of the International Brotherhood of Teamsters, and the Bylaws and laws of his Local Union; to comply with all rules and regulations for the government of the International Union and his Local Union; to faithfully perform all duties assigned to him to the best of his ability and skill; to conduct himself or herself at all times in such a manner as not to bring reproach upon the Union; to take an affirmative part in the business and activities of the Union and accept and discharge his responsibilities during any authorized strike or lockout; that he will not divulge to nonmembers the private business of the Union unless authorized to reveal the same; to never knowingly harm a fellow member; to never discriminate against a fellow worker on account of race, color, religion, sex, age, physical disability, sexual orientation, or national origin; to refrain from any conduct that would interfere with the Union’s performance of its legal or contractual obligations; and at all times to bear true and faithful allegiance to the International Brotherhood of Teamsters and his Local Union."