In 2007, here was how Americans response to some pretty common knowledge.
If Bill O'reilly, a TV host, was allowed to spread wrong concept of how law should be respected, that may explain some of the American's stupidity.
Take a look on the issue of evolution - which is a scientific theory (ie generalisations based on a huge number of observations and can be used to explain a lot of other observations). In 2007, 48% do not believe in evolution. That's is a remarkably high percentage of people with complete lack of scientific literacy. [source]
The American Chronicle's 2006 article pointed out USA became rich because of
The main factor is probably the culture built on Protestant work ethics and solid North European common sense. People work hard and they work smart. They know how to solve problems. While some may call modern Americans "wastrels", the past generations had been very thrifty. The country was built on free enterprise and for hundreds of years the government had a laissez fair policy towards business. The culture is forward looking with a "can do" attitude. People are enthusiastic and believe that they can achieve their individual dreams through discipline and never giving up. Add to this the fact that education is much more accessible to many more people than how it was in Europe and the fact that the society has a much less rigid class structure and you've got yourself a rich country. And don't forget the fact that the country has lots of natural resources and unlike the European countries there have not been any major invasions for the past 200 years or so.
Now that the education system in USA is able to produce 48% of the population who are unable to comprehend the significance of one of the most basic scientific theory, the future of USA is not that bright!