Unfair and/or illegal? Amazon's edition linking policy (stated here) may be unfair to some authors but that doesn't make it illegal. It's true that some unfair business practices violate the law because consumers are deceived. But even if some consumers are misled by Amazon's relevance searches, Amazon's overall rationale for this practice -- it wants to tie reviews of books to specific editions -- seems to have been borne from a desire to prevent consumer deception. In addition, unlike companies who engage in unfair business practices, Amazon does not directly profit from the policy. It sells the same number of books with or without the policy (just not as many of our books!)
Does the policy violate any agreements? Your publisher might be able to challenge the policy if it violates the Amazon publisher distribution agreement -- but that's doubtful because Amazon likely preserves the right to make such policy changes. The policy also doesn't appear to violate the Author Central end-user agreement.
Bottom line dept. Alas, the message from Amazon is that an author jeopardizes goodwill by updating books. There are many workarounds that Amazon could have instigated -- for example, a tag such as "This review refers to a previous edition." And some authors have reported some luck using Author Central's call-in service (while others have not). Jeff, if you're reading (and for some reason we don't think you are), keep in mind that we're also loyal Prime customers!