Please stop reading posting this lie.
Go read the laws, they were not required to pre-finance for 75 year, the 75 years is what is required by the government for calcuating future needs it is for ACCOUNTING purposes only, no money has to saved based on this. The 75 years is used by other governent organizations ranging from IRS, DoD, HUD, CIA, FBI ,etc, etc,etc.
THe post office was required to start pre-funding for the expect life after retiring which is around 20 years. And the reason for that is that the USPS was not planning and just taking money out each year to cover insurance costs for their retires; and with profits decreasing the financial forecast was they would not beable to continue this without taking precaustions. So the real question is why do the people that keep repeating this HATE the retired employees of the postal services that you want to place them in a position where the money need for benefits they worked for are not available?
BTW pull up the actual numbers release by the USPS and those and not the numbers you find in that link you posted.