To pay back its debt all the US government needs is dollars, and hey, look, the US government controls the supply of dollars.
To pay back its debt, the Greek government needs euros, and, oh, the ECB controls the supply of euros. Oops.
If you say the US can't print money because it will cause inflation - well, in the current climate you'd still be wrong, but let's address one issue at a time. The bottom line is that the US and Greece are fundamentally different, and comparisons between them are simply wrong.