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What is the rationale behind stock markets retreating due to S&P having a negative outlook on the USA?
When people (even people in the media) say: "The stock market is up because of X" or "The stock market is down because of Y", they are often engaging in what Nicolas Taleb calls the narrative falacy. They see the market has moved in one direction or another, they open their newspaper, pick a headline that provides a plausible reason for the market to move, and say: "Oh, that is why the stock market is down". Very rarely do statements like this actually come from research, asking people why they bought or sold that day. Sometimes they may be right, but it is usually just story telling. In terms of old fashioned logic this is called the "post hoc, ergo proper hoc" fallacy. Now all the points people have raised about the US deficit may be valid, and there are plenty of reasons for worrying about the future of the world economy, but they were all known before the S&P report, which didn't really provide the markets with much new information. Note also that the actual bond market didn't move much after hearing the same report, in fact the price of 10 year US Treasury bonds actually rose a tiny bit. Take these simple statements about what makes the market go up or down on any given day with several fistfuls of salt.