And the investors who are actually buying US debt don't rely on ratings agencies to give them guidance, and do in fact have expertise in-house to evaluate these things.
As has been mentioned elsewhere on this thread, institutional investors don't treat US bonds like any other, subject to triggers and rules like AAA-only. It's an exceptional case that is treated differently than the others, and one rating agency's opinion is not going to make as big an impact as people seem to think.
It comes down to this: where else would they put their money?
"so they rely on companies like S&P and Moody's to give them guidance."
And those companies fabricate data and sell it to them.
Perhaps not everyone should invest in areas where they have no possibility of understanding the risks.
You just managed to sum up the vast majority of economics problems. Humans are dumb though. Not much you can do.
[1] http://en.wikipedia.org/wiki/Credit_rating_agencies#CRA_busi...
As another comment on this thread pointed out, if the sole maker of the world reserve currency cannot keep a AAA rating, then who really deserves it? As the recent budget fight showed, the US government cannot make more than token attempts to improve its finances. The current debt is about one-fifth of the annual world GDP. Well-nigh invincible power blocs guard the biggest line-items on the budget. Tax increases are off the table and of doubtful effectiveness in this economy even if they were passed.
What confidence should such a government inspire?