The big difference between sugar contained in fruits and that in refined sugar does not lie in its molecular structure. Rather it's what has been taken out of fruit to MAKE it refined....

Refined sugar is bad because it's concentrated. How many sugar cane do you reckon it takes to make a cup of white sugar, or molasse. It would be physically impossible to eat that much sugar cane in one sitting. Our bodies were not designed to eat that much concentrated sugar.

Refining sugar also removes all fiber content which has a big impact on it's GI, as well as removing all nutrients.

It's for the same reason Montignac discourages the drinking of fruit juices. You loose a lot of the nutrients from the fruit, as well as the important fibers. You might be satisfied with eating one orange, but it will take 3-4 oranges to make one glass of juice.

Also one last point on fruits. Did you ever see those little tiny dogs and wonder how the hell they managed to turn them into something so different from their original wolf-like shape. Well keep in mind the same was done to fruits in order to make them sweeter. So fruits now are very different from what they originally where.