Other than a three-day weekend, most Americans don't give much thought to what they're supposed to be observing on Labor Day. In practice, it's become little more than the unofficial celebration of the end of summer. Back in 1882, when New York City's Central Labor Union inaugurated the holiday, the labor movement was quite a different thing than it is today. Once upon a time, workers had valid grievances regarding pay, hours, conditions and safety. Unions served a positive role in leveraging the collective influence of their members to pressure management to redress those grievances. But labor unions are political...