Like everything, I don't see why the federal government should be able to tell you what and whatnot to consume. I like the idea of an FDA to monitor food quality, and to let you know what you're eating, but if someone wants to drink some rat poison, drink some beer, drink some raw milk, or smoke some weed, that's their prerogative.
I think big-agriculture is afraid of small scale farming, local farming, organic farming, etc. It empowers the consumer, which weakens the control of big-businesses. Monsanto doesn't want you to keep your own damn seeds, for crying out loud.