Is S&P Being Sued Because It Downgraded the U.S.?

Why is the Justice Department suing Standard & Poor's for mis-rating structured credit securities before the financial crisis, and not suing Moody's, which gave a lot of the same products the same ratings? I have a theory,1 but Standard & Poor's has another theory, and theirs is a corker:

Well! I mean, lots of people have had that theory. I would link to some of them, but S&P's motion that I just quoted comes with a helpful chronology of all the publications that have hinted at that theory, so you can just read that.2That chronology comes stapled to Harold McGraw's affidavit (McGraw Hill owns S&P) recounting the dramatic story of how Tim Geithner threatened him. It's sort of a nuanced story: