Both sides made a 1st Amendment argument in the case. The non-union teachers saw compelled confiscation of their salary for purposes to which they did not approve an infringement on their free speech rights. The union itself argued that their associational rights were undermined by a law that restricted access to dues previously protected by Supreme Court precedent. The rationale here is that non-members benefit from the efforts of their union and thus cannot free ride on the contributions of their peers.
The majority opinion, authored by Justice Scalia, rejected the union's argument: "The notion that this modest limitation upon an extraordinary benefit violates the First Amendment is, to say the least, counterintuitive." The reasoning, according to Scalia, rests on the premise that "...unions have no constitutional entitlement to the fees of nonmember-employees."
Further dismissing the union's argument. Scalia held, "the contention that this amounts to unconstitutional content-based discrimination is off the mark." In sum, the Court does "...not believe that the voters of Washington impermissibly distorted the marketplace of ideas when they placed a reasonable, viewpoint-neutral limitation on the state's general authorization allowing public-sector unions to acquire and spend the money of government employees." The voters instead "sought to protect the integrity of the election process."
Justice Breyer (joined by Chief Justice Roberts and Justice Alito) wrote a concurring opinion in agreement with the outcome and most of Scalia's opinion absent a few technicalities.
The impact of the decision may be blunted by the fact that Washington modified its law since the advent of the case. It could, however, invite other states to enact similar legislation. As a former teacher who often sought the return of my union dues used for political purposes, I can attest to the fact that it is a cumbersome and elongated process. A check-off provision like that previously adopted in Washington would address this inconvenience while at the same time retaining the collective-bargaining integrity of the teacher unions.