Does it? Many institutional investors don't treat U.S. Treasuries as ratable bonds, but a separate category (they aren't lumped in with AAA corporates in investment strategies, for the good reason that they have quite different characteristics).

Based on the slim case studies we have so far, the S&P downgrade of Japan in 2002 had approximately zero impact on Japanese bond rates. It doesn't even show up as a small blip on the 10-year graph; was just completely ignored.