The problem is guys that the U.S. dollar is the world's reserve currency. You can only buy oil in greenbacks. That forces nations to hold large amounts of them. If the U.S. keeps going into debt (printing bonds that are bought by other nations), then the value of the US$ drops as the money supply expands, making those huge dollar holdings less valuable. The more debt that the U.S. generates, the more America is living beyond its means, and the less faith the world has in America's ability to repay the debt. Someone, somewhere, is going to say "bugger this for a joke" and dump their dollar holdings. If that starts a run on the dollar, then anyone left with US bonds are going to be screwed. However, because oil is denominated in dollars, and everyone needs oil, they can't just buy, say, Euros and ditch the dollar.
Its like standing in a room where everyone is holding a bottle of nitroglycerine. Nobody wants it, but nobody wants to start a panic by throwing their bottle away. So long as no one gets the shakes, everyone's safe.