they don't want to be paid back in worthless dollars...
China warns US on debt addiction; demands global supervision of dollar issues
China, which is the US biggest single creditor, said Washington only had itself to blame for its plight and called for a new stable global reserve currency.
The US government has to come to terms with the painful fact that the good old days when it could just borrow its way out of messes of its own making are finally gone, China's official Xinhua news agency said in a commentary.
After a week which saw 2.5 trillion dollars wiped off global markets, the move deepened investors' concerns of an impending recession in the United States and over the Euro zone crisis.
http://en.mercopress.com/2011/08/08/china-warns-us-on-debt-addiction-demands-global-supervision-of-dollar-issues