For years, opponents of public-sector unions have unsuccessfully challenged the legality of mandatory public union fees. Today, the U.S. Supreme Court overturned a 1977 decision that upheld the constitutionality of so-called “fair share fees,” upending a 41-year precedent that has allowed labor unions to extract fees from nonmembers.The 5-4 decision represents a major victory for Gov. Bruce Rauner, who initiated …