It's the blanket application of rules that are probably based on a sensible situation then extrapolated to all such similar related cases, where they may be far less appropriate.
Similarly you find cough cures with any sugar in them being considered unsuitable for diabetics, ignoring the fact that the viral condition itself has probably already fubared up good diabetic control anyway. I'm not suggesting that you should ignore sugar in a cough cure, but it's just something else that should be taken into consideration, and the amount of sugar ingested is probably relatively small.
It's not sugar that's a problem for an insulin controlled diabetic, it's that the amount of sugar which is ingested should be controlled, and appropriate for the amount and types of insulin being used. I eat sugar (and other carbohydrates) every day, and don't explode because of this.