[...] For more than seventy-five years, the labor movement by law and by custom has been enclosed by and restricted to collective bargaining, with the goal of achieving a contract that seals in wages, benefits, a grievance procedure and specified work rules. In return for that security, workers and their union agree, crucially, to surrender their First Amendment right to withhold their labor. The penalties for violating these unconstitutional agreements are often severe: stiff fines, imprisonment of union officials and sometimes, as after the three-day walkout by New York City transit workers, a court order barring the automatic check-off of union dues.