It's unreasonable to impose a much tougher standard on one and only one organization, even if you think that standard would be good if applied to everyone.
It's not unreasonable from the taxpayers' perspective to require the Postal Service to fund its pension obligations as they are incurred. And this is also not unreasonable from the employees' perspective. When the US government runs out of new creditors (as has already happened to Greece and Italy) employees who are relying on government revenues for their pension will be out-of-luck.
Even if true that it's only the USPS, and yummyfajitas implies that it's actually a broader requirement, here it's a difference between a mistake and not making a mistake. It's not simply an accounting treatment. It's not a defense of the USPS to suggest they should be allowed to make the same mistake that other companies make.
As a business owner concerned about cash flow, I can tell you that paying for anything in advance is always a pain. Reasonable or not, setting aside enough cash to fund the retirement needs of all their workers for 75 years and generating that cash from revenue sounds like a huge pain.
