Eating fruit in it's raw form, involves chewing etc, it therefore takes a lot longer to eat and gives your body enough time to recognise that you have eaten and therefore is more filling than a piece of fruit that has been cooked, pureed or blended etc, where you skip that process completely and don't get the same satisfaction. Not to mention that you have more quantity of fruit in it's raw form. Take for instance 5 apples in their raw form, and imagine how long it would take to eat them, you most likely wouldn't be able to, not without stomach ache anyway. Now stew those apples and the quantity would reduce dramatically and you could probably easily consume those 5 apples in that way. Same goes for blending etc
This question is one that is answered on the SW website FAQ section and it makes perfect sense as to why they have this rule.
It is there to protect every members weight loss. We all need to trust in the plan and follow the rules SW set out if we want to remain 100%, otherwise it isn't the SW plan we are following, it is our own plan and lets face it, that has never worked for any of us before, otherwise we all wouldnt be here trying to lose weight.