Deer are only able to digest food through a mutually beneficial relationship with the microorganisms in the rumen-reticulem compartment of the stomach. Rapid changes in diet can produce digestive disruptions resulting in death, because the microorganisms are not able to adapt to a sudden change in diet.
I've read the same, if corn is being farmed nearby it won't hurt them, if you buy bags from a feed store and introduce it to deer that have never eaten it before they won't be able to digest it and get the nutrients.