According to the actual law, what must be fully funded is the "actuarial present value of all future benefits payable from the Fund." What this means is that if an employee's pension has 50% vested, their "future benefits payable from the Fund" are 50% of their pension. The USPS is then obligated to put 50% of the NPV of their pension into the fund by 2017.
It's explicitly stated 5 times it should comport with "generally accepted actuarial practices and principles", which rules out all the insanity people seem to be attributing to this law.
Is your claim that the postal service must pre-fund unvested benefits at the day the employee is hired? That is, is that how you think "future benefits payable" is being interpretted? Because what's definitely not true is that the postal service is being required to fund un-hired (or unborn) employees. See this article, which has quotes from the Congressional Research Service.