Ah, a common misperception that even many in the industry are confused about. Bourbon can consist of 100% corn, with the only parameter being that it must contain at least 51%. There is no upper limit. Some people will tell you that after 80% corn, the resulting spirit is refered to as "Corn Whiskey." Not so.
Now, recall some basic chemistry. When yeast is added to the mash, what does it convert into alcohol? That is right, sugar. As we know corn is sweet. It is sweet because we have the ability to convert the starches into sugar. Yeast can not do this, hence the primary purpose of malted barley; provide the enzymes that convert starch to sugar so the yeast has something to act upon. Still this is easily overcome by the addition of enzymes during the fermentation process.