Hi Jude, David addresses this issue at great length in both books. The fructose in whole fruit is tempered by the high amount of fibre, which is removed in the juicing process. Also a serve of fruit juice of 250mm, contains far more juice than you would normally find in a single piece of fruit, consequently the 'hit' of fructose is mammoth. So it is both a matter of scale and of complementary bulk.
It is worth reading the appropriate sections in the book again, which can be difficult to fully comprehend in a single reading.
Best of luck
David F