[B]ecause Treasury bonds have always been considered perfectly safe, many rules prohibiting institutions from investing in riskier securities are written as if there were no possibility that the credit rating of Treasuries would be less than stellar.
Banking regulations, for example, accord Treasuries a special status that is not contingent on their rating. The Fed affirmed that status in guidance issued to banks on Friday night. Some investment funds, too, often treat Treasuries as a separate asset category, so that there is no need to sell Treasuries simply because they are no longer rated AAA. In addition, downgrade of long-term Treasury bonds does not affect the short-term federal debt widely held by money market mutual funds.
Also, James Kwak thinks along the same lines as I wrote, but in a far better and more concise way:
S&P downgrading the United States is like Consumer Reports downgrading Coca-Cola. Consumer Reports is a great institution. For example, if you want to know how reliable a 2007 Ford Explorer is going to be, they have done more research than anyone to figure out the reliability history of every single vehicle. Those ratings are a real public service, since they add information to the world. But when it comes to Coke and Pepsi, everyone has an opinion already, and no one cares which one, according to Consumer Reports, “really” tastes better. When S&P rated some tranche of a CDO AAA back in 2006, it meant that some poor analyst had run some model fed to her by an investment bank and made sure that the rows and columns added up correctly, and the default probability percentage at the end was below some threshold. It might have been crappy information, but it was new information. When S&P rates long-term Treasuries AA+, it means . . . nothing. And if any serious buy-side investor were tempted to take S&P’s rating into account, she would be deterred by the fact that the analysis that produced the rating included a $2 trillion arithmetic error.