BG, we should not mix up people or government of the US with US-based rating-agencies. IMO the US government is as much concerned about the european situation as the chinese are. If the whole european economy collaps the US and China have huge problems. At least 20% of the chinese economy depends on Europe, I don't know about the US.
You're right, the US has massive troubles too, e.g. some states are in a similar situation like Greece since a couple of years. But the US is one economy, the EU are 28 economies with one currency - and when one country is in trouble there're discussions for months, not so in the US.