The terms "rock" and "rock and roll" are vague and nebulous. If you doubt this, try looking them up in a dictionary or encyclopedia. And the term "Pop" is, I believe, derived from the word "popular", which in no way indicates that it is not "rock" music. Indeed, to be very popular in the recent past, something pretty much has to be some sort of rock music, broadly defined.
Here is a list of "rock genres":
http://en.wikipedia.org/wiki/List_of_rock_genres
You can read about music genres at:
http://en.wikipedia.org/wiki/Music_genre
I think you will find similar kinds of things at other sources, should you prefer some other encyclopedia.
But, if you wish to use words to mean something other than what most people mean by them, you can do that. You can, if you wish, use terms whose common meaning is vague and imprecise in a manner that is narrowly focused and rigidly defined (by you). But it is foolish to think that others will give up the common way of speaking to adopt the nomenclature you prefer.
I think that an impartial examination of the "Rock and Roll Hall of Fame", and the history of this music, will support the idea that "rock" is a broad category, not a narrow and rigid category. And that Madonna, like her or not, is in that broad category.
