Roberts likewise brushed aside unions' free ridership worries, noting that the problem is "insignificant" where "employees have shown overwhelmingly that they want collective bargaining."

But Edward Dumont, who argued in support of the unions as solicitor general of California, said that sentiment is "a classic collective action problem."

"Many people can want something in the sense they view it as very advantageous to themselves, but if they are given a choice, they would prefer to have it for free, rather than to pay for it," he said.

Frederick said the fees allow for the "shared sacrifice" workers make "to establish a coherent position with their employer" -- which, in turn, benefits the whole group.

But Kennedy couldn't seem to separate that justification, no matter how noble, from the overriding free-speech aspect.

"Many teachers think that they are devoted to the future of America, to the future of our young people, and that the union is equally devoted to that, but that the union is absolutely wrong in some of its positions," he said. "And agency fees require ... that employees and teachers who disagree with those positions must nevertheless subsidize the union on those very points."

Striking his usual pragmatic tone, Justice Stephen Breyer pondered "the court's role in this society" and seemed genuinely worried about overruling a labor law precedent that he acknowledged was not perfect, but was otherwise "a compromise that was worked out over 40 years [ago] and has lasted reasonably well."

When "you start overruling things," he wondered, "what happens to the country, thinking of us as a kind of stability in a world that is tough because it changes a lot?"

A decision in the Friedrichs case is expected by the end of June.

This post has been updated throughout.