There is only ONE reason Amazon removes author reviews.
Authors are the only ones whose accounts be tracked. Fake accounts by authors can still get you some weasel reviews. Indeed, paid reviews can still weasel in under fake names.
Also notice, Amazon exhibits a double standard for its OWN books:
The lesson here is, authors squealed about paid reviews because they were jealous of John Locke, Amazon was forced to react to the bad PR, and Amazon swung the stick at the only thing that could really be cleaned up--author reviews. Sweetly ironic.
This isn't a move to address the rampant review problem, it was designed purely to address the PR problem. If Amazon truly wanted legitimacy, it would limit reviews solely to verified purchasers who use their real names. But Amazon's goal isn't legitimacy--they don't care if someone is writing fake reviews or not (see the NYT piece as evidence--it's okay to write reviews of stuff you haven't even seen, read, or bought), they just want people on their site buying stuff. Authors just happen to be the most disposable and easily replaceable folks they could target.