I think you’re right. Their guidelines say that you must clearly state that you are to ask for a review, whether it be good or bad, and make it clear that there will be no problem if they post a bad review. Asking for a positive review is placing an obligation on them. Asking for an honest review, or better, making it clear that it doesn’t matter whether it’s good or bad, is what Amazon say they want. That seems fair enough, to me. In this case, Amazon is fighting for truth in reviews, against a mass of cheats and people trying to game the system. I was at a talk about self-publishing at the Sydney Writers Festival where a roomful of about 60 people were told to ask everyone they knew to go and post positive reviews of their books on Amazon as soon as they could, to spike it up the rankings. I was so gob-smacked I said nothing at the time; I regretted my silence afterwards. If it happens again, I plan to confront the speaker.

Thanks for your comments, Luke.

I wrote more on this subject in this post:
http://www.justpublishingadvice.com/why-did-amazon-delete-my-book-reviews/

But this is the most telling quote from Amazon’s rule book.
“… family members or close friends of the person, group, or company selling on Amazon may not write Customer Reviews …”

But I did notice a link to an article that both answered my question, and quite clearly contradicts what was probably once true at ‘http://www.justpublishingadvice.com/why-did-amazon-delete-my-book-reviews/’, where you wrote in particular:
“Asking other authors is also a sure way to have a review deleted, as any other author publishing on Amazon is classed as a ‘person selling on Amazon’.”

And the specific information that answered my question was this:

“Paid Reviews – We do not permit reviews or votes on the helpfulness of reviews that are posted in exchange for compensation of any kind, including payment (whether in the form of money or gift certificates), bonus content, entry to a contest or sweepstakes, discounts on future purchases, extra product, or other gifts.
The sole exception to this rule is when a free or discounted copy of a physical product is provided to a customer up front. In this case, if you offer a free or discounted product in exchange for a review, you must clearly state that you welcome both positive and negative feedback. If you receive a free or discounted product in exchange for your review, you must clearly and conspicuously disclose that fact.”

I’ll be testing the accuracy of this out for myself within the next day or so. If I have problems, I’ll let you know!

I agree with you 100% about the evils of paid reviews.