Business advocates see a hidden agenda in these lawsuits. For example, the lawsuit against Schneider—which owns a gigantic warehouse here that serves Walmart exclusively—coincides with unions pressuring Walmart to raise wages. The lawyers and labor groups behind the lawsuit have sought to hold Walmart jointly liable in the case.
Walmart says that it seeks to ensure that its contractors comply with all laws, and that it was not responsible for Schneider's employment practices. Schneider said it ''manages its operations with integrity,'' noting that it had hired various subcontractors to oversee the loading and unloading crews.
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Business groups note that the lawsuits against McDonald's have been coordinated with the fast-food workers' movement demanding a $15 wage. ''This is a classic special-interest campaign by labor unions,'' said Stephen J. Caldeira, president of the International Franchise Association. In legal papers, McDonald's denied any liability in Ms. Salazar's case, and the Oakland franchisee insisted that Ms. Salazar had failed to establish illegal actions by the restaurant.
Lee Schreter, co-chairwoman of the wage and hour practice group at Littler Mendelson, a law firm that represents employers, said wage theft was not increasing, adding that many companies had become more vigilant about compliance. But that has not stopped lawyers from bringing wage theft complaints because of the potential payoff, Ms. Schreter said. ''These are opportunistic lawsuits,'' she said.
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Michael Rubin, one of the lawyers who sued Schneider, disagreed, saying there are many sound wage claims. ''The reason there is so much wage theft is many employers think there is little chance of getting caught,'' he said.
Commissioner Su of California said wage theft harmed not just low-wage workers. ''My agency has found more wages being stolen from workers in California than any time in history,'' she said. ''This has spread to multiple industries across many sectors. It's affected not just minimum-wage workers, but also middle-class workers.''
Many other states are seeing wage-theft cases. New York's attorney general, Eric T. Schneiderman, has recovered $17 million in wage claims over the past three years. ''I'm amazed at how petty and abusive some of these practices are,'' he said. ''Cutting corners is increasingly seen as a sign of libertarianism rather than the theft that it really is.''
In Nashville last February, nine housekeepers protested outside a DoubleTree hotel because the subcontractor that employed them had failed to pay a month's wages. ''The contractor said they didn't have the money, that the hotel hadn't paid them,'' said Natalia Polvadera, a housekeeper. ''We went to the hotel manager—he showed receipts that they had paid the contractor.''
Nonetheless, the protests persuaded DoubleTree to pay the $12,000 in wages owed.
Mr. Weil said some executives had urged him to increase enforcement because they dislike being underbid by unscrupulous employers.
His agency has begun cracking down on retaliation against workers who complain, suing a Texas company that fired a janitor when he refused to sign a statement that falsely said he had already received back wages due him from a Labor Department investigation.
''This is just not acceptable,'' Mr. Weil said. ''You can't threaten people to lose their jobs because they are asserting rights that go back 75 years.''
—By The New York Times' Steven Greenhouse; Dabrali Jimenez contributed reporting from New York.