The reason that was put into place, is that if they don't fund those pensions now, there will be no ability to do so in the future - their business is evaporating.
The business may be evaporating, but the service is going to continue. It turns out that getting documents and packages to any given citizen is really, really important for the function of governance.
In many ways, stripping the post office of their monopoly had the opposite of the intended effect, because it removed the pretenses that let people pretend that it was somehow magically different and special compared to other essential government services just because it sold stuff.
If they held off on paying those obligations until later, they'd be unable to pay them. It's that simple. The math would not have worked out. Reduce the size of their business by 1/3, increase the size of obligations substantially due to employees retiring and drawing against those pensions, and it becomes a very large tax payer funded pension plan.
The USPS has already become increasingly dependent on commercial spam mail to keep going.
