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.
This. Regardless of the technicalities (i.e. if it's the fed reserve, etc. who are responsible for printing money), there's nothing stopping the US government passing a law allowing them to directly print dollars which are legal tender.
As US debt is denominated in USD printing money to pay off debt will always be on the table.
The U.S government cannot print money, only the Federal Reserve can. The government can only get cash from taxes and debt notes, which is why it's important to raise the debt limit.
Actually, it is the Department of Treasury that prints and mints new money. This entity is part of the government, and also manages tax collection etc.
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.
