Many of the minimum wage laws passed in recent years, including in major cities such as Los Angeles, San Francisco and Chicago, include waivers allowing unionized businesses to pay their employees below the minimum. The waivers in effect make unionizing workers a low-cost option for businesses unwilling or unable to pay the new minimum wage, which in cities such as San Francisco and SeaTac, Wash., which also has a waiver, will climb as high as $15 an hour when fully phased in. In those cities, businesses can sign a collective bargaining agreement with a union that sets the wages below...