Here is the link to the ruling: http://www.supremecourt.gov/opinions/11pdf/11-94a1b2.pdf
The Court rules that the case is not moot, first. It then rules on the merits: the union’s treatment of nonmembers who had opted out when they got notice of the dues ran afoul of the First Amendment.
Justice Alito delivered the opinion of the Court, in which Justices Roberts, Scalia, Kennedy and Thomas joined. Sotomayor filed an opinion concurring in the judgment, in which Ginsburg joined. Breyer filed a dissenting opinion, joined by Kagan.
Excerpt Alito’s Opinion: (pgs. 18, 25-27)
…By authorizing a union to collect fees from nonmembers and permitting the use of an opt-out system for the collection of fees levied to cover nonchargeable expenses, our prior decisions approach, if they do not cross, the limit of what the First Amendment can tolerate. The SEIU, however, asks us to go farther. It asks us to approve a procedure under which (a) a special assessment billed for use in electoral campaigns was assessed without providing a new opportunity for nonmembers to decide whether they wished to contribute to this effort and (b) nonmembers who previously opted out were nevertheless required to pay more than half of the special assessment even though the union had said that the purpose of the fund was to mount a political campaign and that it would not be used for ordinary union expenses. This aggressive use of power by the SEIU to collect fees from nonmembers is indefensible…
…Which side should bear this risk?
The answer is obvious: the side whose constitutional rights are not at stake. “Given the existence of acceptable alternatives, [a] union cannot be allowed to commit dissenters’ funds to improper uses even temporarily.” Ellis, 466 U. S., at 444. Thus, if unconsenting nonmembers pay too much, their First Amendment rights are infringed. On the other hand, if unconsenting nonmembers pay less than their proportionate share, no constitutional right of the union is violated because the union has no constitutional right to receive any payment from these employees. See Davenport, 551 U. S., at 185. The union has simply lost for a few months the “extraordinary” benefit of being em- powered to compel nonmembers to pay for services thatthey may not want and in any event have not agreed to fund.
As we have noted, by allowing unions to collect any fees rather than opt-in schemes when annual dues are billed, our cases have substantially impinged upon the First Amendment rights of nonmembers. In the new situation presented here, we see no justification for any further impingement. The general rule—individuals should not be compelled to subsidize private groups or private speech—should prevail.
Public-sector unions have the right under the First Amendment to express their views on political and socialissues without government interference. See, e.g., Citizens United v. Federal Election Comm’n, 558 U. S. ___ (2010). But employees who choose not to join a union have the same rights. The First Amendment creates a forum in which all may seek, without hindrance or aid from the State, to move public opinion and achieve their politicalgoals. “First Amendment values [would be] at serious riskif the government [could] compel a particular citizen, or adiscrete group of citizens, to pay special subsidies for speech on the side that [the government] favors.” United Foods, 533 U. S., at 411. Therefore, when a public-sectorunion imposes a special assessment or dues increase, the union must provide a fresh Hudson notice and may not exact any funds from nonmembers without their affirmative consent…
For those unfamiliar with Knox v. SEIU, you can catch up by listening to the Supreme Court’s oral arguments below.
Suggested reading: Supreme Court to hear NRTW case against SEIU’s polical $$$ grab from unwilling workers