The inner class is just a way to cleanly separate some functionality that really belongs to the original outer class. They are intended to be used when you have 2 requirements:
- Some piece of functionality in your outer class would be most clear if it was implemented in a separate class.
- Even though it's in a separate class, the functionality is very closely tied to way that the outer class works.
Given these requirements, inner classes have full access to their outer class. Since they're basically a member of the outer class, it makes sense that they have access to methods and attributes of the outer class -- including privates.