But what is this system trying to secure against?

America already grows enough animal fodder without counting corn for ethanol. If some calamity strikes corn production for animal fodder, it will equally affect corn production for ethanol. Because it's the same crop.

And also why can't you scale farm production up and down? It isn't like manufacturing and factories. Preserve farmland and produce enough for the country's consumption needs. That'll keep farm labor and machinery sufficiently busy. It also prevents the waste of fertile soil growing food that's never eaten.