This may be painful to hear, but the rule is a good one. The effects of hypoglycemia can be quite insidious... they can sneak up on a diabetic rapidly, with little or no warning, and rob you of the ability to even comprehend what is wrong with you.
Obviously, the same thing can happen to you while driving a car, but the effects would obviously be more dramatic at the controls of an airplane. Throw in crazy hours, crazy eating patterns, time zone changes etc, and a professional flying career (at least as an airline pilot) is not a great idea for an insulin dependent diabetic.