Batgrl comment: Passing this along for anyone who didn't realize there's now a box to check about sharing your reviews. apparently that box has been there for some time. Since it's checked by default if you don't want your GR reviews used on Amazon's website, you have to uncheck the box. Update: GR may share your reviews on Amazon anyway. Dammit.
How to: in GR go into Edit Profile, choose the Settings tab, scroll down til you find the box "allow partners of Goodreads to display my reviews"
And now, the reblog:
I actually got an answer from Kara saying "All Goodreads reviews will display on Goodreads on Kindle, as it is part of Goodreads. This is similar to how all reviews show on the Goodreads iOS app and the Goodreads Android app. It's worth noting that you have to be a member of Goodreads to access Goodreads on Kindle, which includes the Goodreads book pages.
As Elizabeth mentioned, the third-party sharing box will be grayed out by default if your profile is private. If you set your profile to private, we automatically set your reviews to only appear on Goodreads." (Emphasis mine)
I have also asked in additional to the original post Kara answered if private profiles being set to "only appear on Goodreads" meant "only appear on Goodreads and the kindle now that kindle is part of Goodreads" -- I was a bit surprised to get a quick answer or I might have phrased that better. I plan on updating post when answered (or if you know how flaky I can be, click the source link to check for yourself).
Screenshot of original post/question: