First it is not illegal to feed and hunt in all areas, only some. Google "deer feeder" there are industries around this market.
I discuss some of the considerations in my answer at What are the downsides of baiting game while hunting?
Where food is grown, grown food plots are always present, all animals including game animals come and go to these plots as they desire. It just in not practical to say you can't hunt in the crop fields of the US grain belt. Additionally it takes substantial work outside of the hunting season to maintain this plot of land.
If a hunter grabs a bag of apples at the store, and drops them in a field, the food is only there by the direct intervention of the hunter. True he probably does it more than once, but the point is that is unnatural. The food surplus (what the animal eats), is greater than the environment can support.
The real short answer is: Where baiting is illegal, the law makers believe it will increase the harvest rate (killing) of target species above what the ecosystem can sustain.
There are environmental, political and economic reasons that make the extermination of game animals in some areas undesirable. Hunting laws are generally designed to maintain stable population and harvest rates.