It's been a difficult June for President Obama's allies in organized labor. First, Wisconsin Gov. Scott Walker easily beat back a union-funded recall election, ensuring the survival of his reforms to collective bargaining and other public-sector union privileges. In Virginia, the bureaucrats controlling the nation's largest ongoing public works project -- the Silver Line to Washington Dulles International Airport -- were forced by political pressure to drop union contracting preferences that had threatened to exclude firms from Virginia, a right-to-work state.

And then this week, organized labor received yet another blow in the form of the Supreme Court decision Knox v. SEIU. The 7-2 ruling blocks public employee unions from padding their political coffers with contributions from unwilling no-members. It applies to closed-shop states, such as California, where the government deducts union representation fees even from the paychecks of nonmembers.

In 2005, the SEIU Local 1000 in Sacramento leveled a special assessment explicitly to support political activity, without giving nonmembers a chance to opt out. In fact, the union used part of those nonmembers' money to fight a 2006 ballot proposition that would have safeguarded those same nonmembers' right not to fund union political campaigns.

The 7-2 majority -- which included a concurrence by liberal Justices Sonia Sotomayor and Ruth Bader Ginsburg -- agreed that the union had acted illegally. But the sharpest cut comes from the court's narrower 5-4 majority opinion by Justice Samuel Alito. It held that for unions to deduct special political contributions from nonmembers' paychecks -- as occurred in this case -- those workers must explicitly opt in, as opposed to having to opt out. Otherwise, as in this case, the union will at best receive a free loan for political activity at nonunion employees' expense. At worst, employees burdened with the normal concerns of life may well forget to claim their full rights, and the unions' political activities will thrive by default at their expense.

"Public-sector unions have the right under the First Amendment to express their views on political and social issues without government interference," Alito wrote. "But employees who choose not to join a union have the same rights." It should come as little surprise that nonunion workers do not want to contribute to union political initiatives, especially ones that attack nonunion workers' rights.

Labor unions provide a service to their members -- a service that is worth whatever those members are willing to pay, but not a cent more. The Supreme Court's ruling affirms this valuation. It continues a welcome trend across America of plugging up and sealing off unfair revenue streams that public-sector unions, over the years, have managed to extract from the unwilling -- from taxpayers, governments and nonmembers.