RIDERS, LEGISLATIVE, are sections or clauses not germane to the subject matter of a bill that are added by amendment before passage for a specific strategic purpose—namely, that the sentiment favorable to the bill will be sufficient to sweep the whole enactment through the final vote and secure executive approval, even though the proposal would probably be defeated by vote or vetoed if submitted separately. For example, in 1913 Congress attached to an appropriation act a rider exempting labor unions from the Sherman Antitrust Act. President William Howard Taft had to veto the whole bill to annul this provision.
Luce, Robert. Legislative Procedure: Parliamentary Practices and the Course of Business in the Framing of Statutes. New York: DaCapo Press, 1972.