The assumption is there is no good reason for the US to default, as it can always print more dollars and pay debt with them. In contrast, Greece can't print euros by itself.
As US debt is denominated in USD printing money to pay off debt will always be on the table.
Though you are right that the U.S. government can't print & spend directly. The printed money has to be brought into circulation by the Federal Reserve, which lends it out at the Fed rate. Then again, the Federal Reserve's profits go back to the Treasury so the government can in practice borrow the printed money from the Fed without interest.