I think of it in terms of how much you could gorge on!! as the general principles are guided by appetite satisfaction. I could probably eat super amounts of puree'd apple or other fruit but couldn't possibly eat so much of any puree'd veg or even veg soup (and I love soup!). So it's a rule to limit you from over-eating.
This came up in my last meeting and my C said it was due to the natural sugars in fruit and how our bodies break down that sugar. Apparently if you cook or puree fruit it breaks down the sugar so that our bodies do not have to. It does not apply vegetables due to their low sugar contents.