In Abood v. Detroit Bd. of Ed., 431 U. S. 209 (1977), the Supreme Court recognized that even in matters directly related to collective bargaining, compulsory funding of union activities has an impact on employees' First Amendment interests, since the employees might disagree with positions taken by the union on issues such as the inclusion of abortion in a medical benefit plan, or negotiating no-strike agreements, or even the desirability of unionism in general. (431 U. S., at 222.) To be sure, the Supreme Court concluded that any interference with such interests was "constitutionally justified by the legislative assessment of the important contribution of the union shop to the system of labor relations established by Congress." (Ibid.)