This isn't just a Bush Casualty. Decades union benifit agreements were made. Workers said "We want more money" and politicians, to kick the can down the road said "We can't give you money now, but we'll give you sweet retirement deals". The unions thought it was a good deal. It was sold to the American people as a "no-cost" benefit. Politicians got away with it because the state and federal budgets weren't constrained to GAAP (Generally Accepted Accounting Principals).
In particular, the "Principle of periodicity" they skirted state that future benefits and payouts must be fiscally accounted for, incrementally between the date of the agreement and the date of the disbursement. Think about it, if I were a business owner and promised you, my employee, $1,000,000 bonus if you contingent upon you working for me till retirement age of 65. We agree and I do nothing in my accounting books to account for that $1,000,000 in the ensuing 40 years of labor. How will I have the money for you when you retire? That makes my agreement with you, at the very least, an empty promise... more likely, it's a breach of contract.
Now, considering that you are not allowed to sue a government agency, and considering that many of the politicians that employed this practice have retired themselves (with their own self-appointed retirement packages), the current government has no choice but to do what it should have done all along, follow GAAP. To the best of my knowlege, they JUST started doing this under Bush even though GAAP has been around in some form for over 70 years (Please check this statement).
So why do USPS competitors not carry the same burden? Simple: If they are a publicly traded company, SEC requires them to follow GAAP in their operating reports. In other words, no agreement was made that went unfunded AND any agreement made has been incrementally ACCOUNTED for DECADES. The American people should be HOPPING MAD that we allowed ourselves to be fooled like this.
"Because there is no such thing as a free lunch."