The point being, you build bacteria in your mouth during your sleep. So when you wake up and immidately eat, those bacteria have alot of new stuff to develop on. And if you drink juice or eat apple/orange/pineapple the acid will stay and be rubbed in when you start brushing (you have 2 wait 30 min. With brushing after that stuff, which no one does). Furthermore you have all kind of crums in your mouth after eating, you want to brush with that there?
If you brush you teeth when you wake up, you remove the bacteria and you be good to go. And it it always a good idea to drink water after a meal.
So people who knows why they brush their theeth and when is good to do it, do it when they wake up.