People naturally reject anything that doesn't dovetail with their current choices. And the time-honored "You are what you eat" is a prime example.
Canada's CBC has a great show called Marketplace that reveals how food corporations — with the approval of the US' FDA, Canada's CFIA, and similar government agencies worldwide — defraud us by promoting the labeling of their products as "healthy," when in fact they are a danger to our health.
"Garbage in, garbage out" certainly applies to the diet we choose, so those who expect quality muscular gains from hard workouts that are followed by a stop at McDonalds or Pizza Hut have to realize how dumb that is.