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.
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.
The USPS has already become increasingly dependent on commercial spam mail to keep going.