The AbstractIteratorDecorator defines a getIterator method, which is protected, thus only intended to be used by derived classes, not by users. This is similar to the collection-type decorators, where the corresponding decorated() method is usually also protected.
For our iterator decorators it is less likely that somebody will extend them, so there is less of a need to provide access to the underlying iterator, and I fail to see a use-case where a user decorates an iterator and later on wants to retrieve the decorated iterator again (and would rather design the API in a way to prevent such usage).
That said, there are now some iterator decorators, like Unmodifiable that provide access to the decorated iterator, and some of the newer ones, like PeekingIterator, that dont. I would prefer to not provide access at all and deprecate the existing getIterator() methods for future versions. There are some weird things in the API that were added a long time ago, and I did not want to change everything when doing the 4.0 release, but we can decide to gradually improve the API and deprecate things that do not make sense anymore.