This idea is based on the fantasy idea of "it's fine because the US can just inflate away its currency to reduce its debts". This is also often used as an argument for why countries shouldn't join the Euro because they'd be giving up an important tool.
The reality is that purposefully inflating your currency to reduce your debt burden is going to upset your creditors just as much as if you just defaulted on your debts, but will have the added affect of crippling your economy with inflation. Look at how much Americans freaked out over a year or two of 6% yearly inflation. How do you think Americans would respond to 30% *monthly* inflation like in Argentina or Turkey?
It's not like lenders won't notice if the USA tried to print its way out of debt.