So do all private sector businesses. You can't promise employees $X in future benefits without putting $X into a pension fund. This requirement is uncommon in government, which is why many municipalities have unfunded pension obligations, but it's completely reasonable. The scary fact is that we only require the USPS to do this.
You are also misrepresenting the 75 year time horizon. The USPS is required to make the following (wildly oversimplified) spreadsheet:
year #living employees # earned costs/employee
2014 1000000 50000
2015 900000 51000
etc
The pension fund needs to have SUM(column B x column C) dollars in it (again, wildly oversimplified - it's actually the Present Value of Future Benefits). The 75 year requirement means the spreadsheet must have 75 rows. This prevents the fund from cheating by cutting off the calculation early and failing to account for payments they promised to make. Changing 75 years to 100 years wouldn't change anything.The number "earned costs/employee" is the fraction of the pension costs that have already vested. I.e., if the employee has earned a pension of $100/month so far, but will have earned a pension of $5000/month at retirement, their earned costs are $100/month, not $5000/month.
This is the same calculation that ERISA requires of all private sector companies.
(Certain grandfathered companies in the private sector are also allowed to escape ERISA, and this will be a problem if any of them go out of business.)