I'm guessing the doctors just tell all diabetics not to fast in order to cover for themselves that they don't inadvertently harm someone who does something unwise if the doctor made room for exceptions, as in better safe than sorry/abundance of caution/don't get sued etc. And some doctors believing the advice not to fast because that's all they've heard. I used to have hypoglycemia, was told to skip even my monthly religious/spiritual fast day, eat lots of times throughout the day and so on. Definite low blood sugar symptoms if I didn't. Now with very low carb and body relying on fat for its fuel I do my monthly spiritual fast and skip meals whenever desired with no problem beyond a grumbling GI because it enjoys being put to use. With on-board fat for fuel it's not possible to run out of energy in that amount of time. Good riddance to the sugar rollercoaster, having to tote carbohydrate snacks, etc. Also learned over the years my hypoglycemia as a younger person was a warning sign for future T2 diabetes. Dietary changes are keeping that under control, too.