The main argument used by critics of right-to-work laws is that they create a what economists call “a free rider” issue. That is because since a union negotiates on behalf of all employees in a workplace, non-members also get the benefits the unions negotiates without having to pay union dues.

In an op-ed for the Detroit Free Press, Rep. Sander Levin, D-Mich., even said that Michigan Gov. Rick Snyder was creating “a right to freeload.”

It is a somewhat compelling argument, but critics like Levin obscure an important point: unions are not in fact obligated to represent every worker in a workplace. A economist James Sherk of the conservative Heritage Foundation points out, unions can negotiate “members only” contracts with employers:

The National Labor Relations Act does not mandate unions exclusively represent all employees, but permits them to electively do so. Under the Act, unions can also negotiate “members-only” contracts that only cover dues-paying members. They do not have to represent other employees. The Supreme Court has ruled repeatedly on this point. As Justice William Brennan wrote in Retail Clerks v. Lion Dry Goods, the Act’s coverage “is not limited to labor organizations which are entitled to recognition as exclusive bargaining agents of employees … ‘Members only’ contracts have long been recognized.”

Unions prefer not to negotiate these kinds of contracts precisely because they alert workers to the fact they don’t have to belong to a union. By not being the exclusive employee representative in workplaces, the unions lose leverage with employers as well as the dues from those other workers. So, unsurprisingly, they’d rather keep the conversation focused on exclusive contracts.