The difference being that a pension is for a future event. When a state/federal pension can't meet its obligations, which were exaggerated and based on early 1980s T-bond yields, it doesn't fold and go away. It just raises taxes.
Of course, when a corporate pension becomes "overfunded," the company borrows from it. Like Social Security, in many respects.
And that actually is a good point. Now a law that WOULD be fair, is to say that if a pension is promised as part of your pay, then they need to keep X percentage of it in liquid assets. There are already laws on the book in most states about companies by law having to pay you what was offered. Stopping a company from hiring you for X dollars and then paying you Y dollars, or not paying your your vacation time when you leave the company.