It's the difference between what being in ketosis means for a non-diabetic person and a diabetic person.
A non-diabetic person will only go into ketosis if he/she has been starving for a few days (i.e., eaten nothing), or has been severely restricting carbohydrate intake. The body then quite normally and quite naturally switches its main fuel supply from the burning of carbs to the burning of fat. Our bodies are designed to do this - in fact, our paleolithic ancesters probably spent most of their time in ketosis (no flour, sugar or potatoes around back then
).
On the other hand, if you're eating a normal diet (that's one with the usual mixture of carbs, protein and fat in it - especially carbs) but you find ketones in your urine it's not a good sign. It generally means that your body is unable to burn the carbs you're feeding it, the unused sugar is sloshing about in your bloodstream, and you've had to switch into fat burning mode to make energy. That's a classic feature of diabetes - hence the reason ketostix were invented, to spot ketones in the urine.
It can be useful to know you're in ketosis if you like having that reassurance, but quite honestly, if you follow the diet 100%, you'll be in ketosis within 3 or 4 days of starting.
