Are magazines republishing?

In the past months I have read two Rolling Stone articles that take exclusive rights of the excerpts from the memoirs of Keith Richards and Sammy Hagar. While the microscopical sections of their memoirs included in the magazine are entertaining and almost serve as a larger than life nut graph to make you buy the actual book, I wonder if it's entirely ethical.

Choosing to include an excerpt might mean several things: the staff at RS might have been short of one article, the publisher of the memoir is paying RS to sort of advertise the book in their magazine, or RS genuinely feels that the writing is truly masterful and goes with the guidelines that they would require out of their writers.

I like to think that the third option is the correct one even if it makes me sound naive. But at the end of the day, isn't doing this plagiarism?
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The material is already written and is going to be published. RS publishes the excerpt a few weeks in advance before the book is released. SInce the author and publisher of the memoirs are allowing the use of their memoir to be published, it obviously isn't considered plagiarism because RS is giving full credit to both the author and publisher. But isn't it ripping out the readers of RS?

A simple review would suffice to make me one to buy the book or not. Instead, they take up space from excellent profiles to stick in four to five pages of memoirs. I enjoyed Sammy Hagar's excerpt because it was short and condensed, and straight to the whole Van Halen issue. Richards', however, was a cover story, excessively long and unorganized.

At the end of the day it depends on what the readers like. Care to share?