US treasury and the world.

I have a question or two on treasury, federal reserves and other countries.
Question: Why do other countries such as South Korea, China, Japan etc invest their money in US treasury? and hold US $s in their reserves as well. Does US do the same? To me this comes across as if the countries investing in US treasury are weak and dont trust their own economies. Why dont these countries invest in their own countries?

http://www.nytimes.com/2005/03/11/business/11dollar.html?