Congregating the deer is part of the problem, but the main thing they are trying to stop is the potential transfer of a disease through the feed. Many/most manufactured feed has animal byproducts. Those byproducts can be the source of the disease (ie - infected animal is killed and its parts put into the feed, deer eats feed, .......). That is the main reason for the feeding ban. Now corn grain and other natural feed, obviously doesn't have animal byproduct, but it is banned nontheless.