Why does anyone buy insurance policies that pay off only if the U.S. goes bankrupt?

It's an interesting question and story raised by Newsweek on the practice of buying insurance on U.S. government debt. It is is a scenario where there is only profit if there is a failure and in the case of the US, the valid question of who would even pay it is asked:

According to the Bureau of Public Debt, there is $8.15 trillion in U.S. government debt owned by the public. In addition, now that the United States has taken control of the failed mortgage giants Fannie Mae and Freddie Mac, the government is formally standing behind the debts of those two entities, which surpass $5 trillion. Now, let's imagine a world in which the U.S. government, lacking the will to tax or cut spending, can't scrape up the cash to stay current on interest payments and can't roll over debt as it matures. That would trigger a huge decline in the value of treasuries and mortgage-backed securities. The balance sheet of every U.S. financial institution—JPMorgan, Goldman, Citi, your neighborhood bank, the Federal Reserve, money-market funds—would be decimated. There wouldn't be a single solvent bank, insurer, or company in the United States. The large multinational banks, which have significant U.S. operations and plenty of this stuff on their books, would likewise be wiped out. Oh, and foreign holders of U.S. debt—see this list topped by China and Japan—would be toast, too.

In this dystopia, who, precisely, would be able to make good on the insurance sold on U.S. government debt? The last time we had a set of events that were supposed to trigger large-scale payment of credit-default swaps, the system basically shut down. All the investors who bought insurance on financial instruments from AIG got paid off in full only because the U.S. government bailed the company out. Who would bail out the Treasury Department and the Federal Reserve?