There are things that are true because the facts prove them to be so. Then there are things that are true because the majority of people believe them to be so. An example of the first is, "water freezes at 32 degrees Fahrenheit". An example of the second is, "a United States Bond is 100% guaranteed to be paid."
As the US currently owes approximately 17 trillion dollars and services that debt by selling yet more debt, it is not an unimportant matter that people believe the government is a good credit risk.
Though it is very likely the government will forestall default, they are indeed playing with fire by raising even the slightest concern about their creditworthiness.
The moment a significant number of people lose confidence in America's debt is the moment that the debt will, like the start of an avalanche, begin to fail on its own.