Last month, the Supreme Court’s agreed to review Janus v. American Federation of State, County, and Municipal Employees, Council 31 (Cato filed a brief in support of the plaintiffs). The case is a First Amendment challenge to the “agency fees” that must be paid to a public-sector union by non-members. As a matter of existing First Amendment law, no employee may be compelled to join a union or contribute money to fund a union’s direct political activities, such as political ads. In roughly 22 states (the 28 “right-to-work” states outlaw agency fees), unions may compel non-members to pay agency fees that (ostensibly) only reflect the cost of the union’s representational activities, such as bargaining over wages and working conditions. The agency fee is the product of the Supreme Court’s decision in Abood v. Detroit Board of Education (1977), in which the Court prohibited public-sector unions from compelling non-members to support political speech, but allowed for the compelled support of the union’s other “non-political” activities.

The plaintiff in Janus—like the 2015 Friedrichs case that stalemated after Justice Scalia’s death (in which Cato also filed a brief)—claims that, for public employees, the distinction in Abood between “political” and “non-political” is illusory because the terms and conditions of public employment are inherently a matter of public concern. A teachers union negotiates with a school system over salaries and benefits packages, merit pay versus seniority, the standards for teacher evaluation, and the controversial “tenure” provisions that in some states make it nearly impossible to fire even serial abusers. Each of these represents a core, political issue in education policy, and a teacher who believes that, say, merit-based pay systems would improve the quality of teaching in the school system (where perhaps her own children may attend) can currently be forced to fund negotiations against it.

Abood upheld the agency fee based, in part, on the “free rider” rationale. The Court reasoned that, since unions are required expend resources for dissenters’ benefit, dissenters may be required to cover that expense because otherwise they would get a free ride on the supposed union gravy train. Recently, in Slate, attorney Daniel Horwitz—drawing on the argument of a pair of law professors—took the issue a step further, claiming that forcing unions to represent free riders is unconstitutional. Horwitz argues that unions should not have to abide by the duty of fair representation—meaning they have to fairly represent the interests of both members and non-members—if non-members are not made to pay (that is, if Mr. Janus wins). But unions aren’t compelled to abide by the duty of fair representation; they choose to when they become the exclusive bargaining representative.

There’s a mistake Horwitz makes that’s even more basic: he seems to think public employees are covered under the federal National Labor Relations Act. But the NLRA covers only private-sector employees, who would be unaffected by Janus. State and local public employees like Mark Janus are not covered by federal labor law at all (federal employees are covered under a separate civil service system). The actual effect on the rights of unions will therefore come in 51 permutations. But most state laws are analogous on the relevant points, so let’s proceed on that simplifying assumption (with the understanding that any particular state may vary).

Currently, a union that receives a majority of the votes of the relevant group of employees in an election may be certified as an “exclusive bargaining representative.” This provides the union certain rights, including imposing a duty on the employer to bargain in good faith with the certified union, and excluding competing unions and dissenting individual employees from the bargaining table. In return the union must shoulder a “duty of fair representation,” which requires the union act as a fiduciary to protect the interests of both members and non-members.

But unions can be members only, thus eliminating all the problems that come with forcing people to contribute to a union. In the words of two prominent labor scholars (one of whom is cited by Mr. Horwitz):

Unions don’t usually pursue members-only representation because becoming the exclusive bargaining representative of all employees grants special privileges, namely an employer’s affirmative duty to bargain. Only a union that is certified as the exclusive bargaining representative must shoulder the burden of non-members. This makes sense. A member of Congress could be thought of as the “exclusive bargaining representative” of his constituency, which means he has a duty to represent both those who voted for him and those who did not. Non-members of a union are like those who voted for the other guy.

However, nothing compels a union to become the exclusive bargaining representative. In fact, the “exclusive bargaining” model is an American peculiarity, born of the particular zeitgeist of the 1930s. In Europe, unions are members only, with multiple unions in a given workplace, often tied to a particular political party or identity. Workers therefore gain the opportunity to associate with a union that represents all their interests, and champions the causes they value. And, as any American who has traveled Europe and encountered a transportation strike knows, European unions are pretty powerful.

For many workers who dislike unions, compelled support is often their biggest objection. Also, for many libertarians and First Amendment devotees, unions are only objectionable when they’re coercive and non-voluntary. In the long run, a decision for Mr. Janus could help unions move towards a potentially more popular members-only model.