The US gov't sure as hell requires private corporations with pensions to make sure they are fully funded. The pension might be optional, but running an underfunded one is not.
"AFTER years of poor investment returns, the pension funds of the United States’ largest companies are further behind than they have ever been.
The companies in the Standard & Poor’s 500 collectively reported that at the end of their most recent fiscal years, their pension plans had obligations of $1.68 trillion and assets of just $1.32 trillion. The difference of $355 billion was the largest ever, S.& P. said in a report.
Of the 500 companies, 338 have defined-benefit pension plans, and only 18 are fully funded. Seven companies reported that their plans were underfunded by more than $10 billion, with the largest negative figure, $21.6 billion, reported by General Electric."
Private corporations are not held to the same standard as the USPS for pension funding.
As the article you linked says, they're underfunded because the investments didn't perform as well as expected.
If there's something else to it, by all means feel free to state precisely the difference between how these pension funds from the random article you googled operate, and the USPS's requirements. There might be, but the article you linked doesn't imply that.
