I think there's a difference in an author saying 'you interpreted my book wrong' and clarifying something implicit but unmentioned in the text. For me, the former is unjustified and specious arguing by the author, but the latter may be permissible as long as it doesn't completely screw with people's interpretations of the themes/subtext and the like.
I think you just brought up what I was hoping to get at: what happens when the author's interpretation clashes with the reader's? Like Kyoko said, we have impressions of the book and if the author tries to change that after the book's been written that can be difficult for us readers.
But then there's the flip side. Authors don't want their books hijacked. To bring up HP again, a lot of people called the books evil and full of devil-worship and all of that. As an author that's got to feel horrible having other people tell you what your own work is about, even if it's not as extreme as what happened with HP.
My own view is that the readers should be able to interpret however they want even if some of them make wildly unsubstantiated claims (like witchcraft) because accepting only what the author says a book is is too limiting.