On Tuesday an airplane buzzed the headquarters of Standard & Poor’s in lower Manhattan, trailing a streamer that read “Thanks for the downgrade. You should all be fired.” It’s a common — and understandable — enough sentiment these days. Over the past decade, S&P and its rival Moody’s helped bring on the financial crisis by completely flubbing their evaluations of subprime mortgage securities and the derivatives built atop them. They gave AAA ratings to financial instruments that were essentially fraudulent to begin with and effectively worthless at the end. Now one of these agencies has the unmitigated gall to downgrade the U.S.?!?!