Referring to unboxing/autoboxing, you have to imagine them like are two ways that the compiler adopt to save you from going mad with continuos "casting" from primitive to object and vice-versa, but they are not flawless.
What happens if your Integer wrapper is null and you do a division? Not a division by 0 but a Null pointer exception, because java cannot unbox a not referenced object!
So it's safe and logical to keep different init rules for primitives and objects.
