Why ridiculous? The treasury cannot, by law, issue more than a certain amount of debt. The politicians may change the law - but that's a vote that's yet to have been taken. Once upon a time, it would have been unthinkable for Japan to have had a lower credit rating than AAA.
Also, the US owes money in currencies other than $, which the Treasury doesn't have the power to print.
Mind you, despite all this you can have a serious news piece of "someone from Moody's" claiming that they might be forced to lower the US' rating http://www.bbc.co.uk/news/business-14142621 - which frankly just sounds all kinds of ridiculous.