USPS is NOT put in a worse situation than private companies. It had a privilege that put it in a much, much better situation revoked.
The fact that future governments can (probably) be relied on to make good on promises made today is a "weak argument" for just kicking the can down the road instead of funding the liability immediately.
In 2017, USPS's costs will go down, significantly, because they no longer have the backlog of pensions to fund, they will just to funding pensions on an ongoing bases. Exactly like private companies. At that time, USPS will be in a situation where it can be reformed, because it no longer carries around a glut of unfunded liabilities that will be dumped in the government's lap if something goes belly up.