In fact, Apple(look at the nutritional values): http://en.wikipedia.org/wiki/Apple - Sugars 10.39 g - Fat 0.17 g
Peanut: http://en.wikipedia.org/wiki/Peanut - Sugars 0 g. - Fat Saturated 9 gr!! - Fat monounsaturated 24gr!! - Fat polyunsaturated 16gr!!
Maybe that doesn't tell you anything but 100 gr olive oil has more calories that 300gr of sugar.
Eating apples is natural for human beings,like many other fruits, eating peanut butter is not(apple has a lot of water with their sugar what makes it easy to process, and converting its sugars to fat would take energy).
They are other reasons for storing fat, e.g cold protection and cell and arteries protection from acids(produced by sugars burning).
The calories approach is an oversimplification.