One might suggest that Greek/Argentinian/Venezuelan bonds aren't risk-free. :)

It's true that the US Govt could print its way out of nominal debt, but there are market constraints on how aggressively it may do so if we want the system to keep working, because stable inflation is a key component in the demand for government bonds. Once inflation stops being stable, your government's ability to issue new bonds is in trouble, because buyers demand higher yields to encapsulate the risk of that uncertainty.

I'm of the opinion that reality is somewhere in the middle - US government debt isn't the same as household debt, but it's not meaningless, either, and I think it does everyone a disservice when we discuss it as being characteristic of either extreme.