Few workers would turn down a raise. Union members, however, can have raises turned down on their behalf. Employees of a Pennsylvania grocery store learned this the hard way. Managers at the Giant Eagle grocery in Edinboro, Pa., wanted to reward hard work. So they boosted the wages of two dozen high-performing employees above their union rates. But United Food and Commercial Workers Local 23 was not pleased. The union argued the pay increases violated their contract, took Giant Eagle to court and forced it to rescind the raises. Why did Local 23 oppose higher pay for its members? Because...