Not a nutrition expert, but I'm given to understand that the form of the sugar matters as much as the amount. The sugar in an actual apple is apparently bound up in fibers or something, so basically it takes time for your body to process it and get it into the bloodstream. When you turn that apple into apple juice, even if it's entirely natural, you've freed the sugar to be absorbed by your body much faster.

That speed of absorption makes the difference - your body doesn't know what to do with so much sugar at once, so it processes it into fat and other things. Let it out more slowly, and it can actually use most of it for energy.