I remember when I first started Crossfit and decided to try out the Zone. I would record my foods for the day and be shocked at the low number of calories (like 1200 for a 180lbs female). When I and others asked about this on the Crossfit forums, everyone chimed in with "Oh, your body will just get used to it" and "Humans are too accustomed to eating too many calories anyway, we don't really need that many!" The euphoria produced by eating the Zone for long periods of time was also another argument towards the diet.
Of course, looking back these claims were utterly ridiculous and the "euphoria" is likely nothing different than the euphoria anorexics also report after extended periods of starvation. But at the time I figured people knew what they were talking about (always a poor assumption on Internet forums).