"Even [the DOL] admits that its ERISA exemption is dubious. Under a 1975 Labor rule, employee benefit plans are exempt from ERISA only if they are 'completely voluntary.' ... [DOL] is overturning its own precedent on the conceit that a state government that sets 'the terms for and administers a payroll deduction savings arrangement ... is far different than when the employer' does. Yet the Supreme Court, as Labor notes, has concluded that ERISA pre-empts state laws that 'mandate employee benefit structures or their administration.' "