Well, I think we all make social sacrifices for the benefit of a well run society. While we don't always get a well run society, we can at least comfort ourselves that our sacrifice might make a SLIGHTLY better run society.
Those authors who don't like this modest restriction on their 'rights' as an Amazon customer in order to make the reviews more credible, should probably find another publisher or post their reviews on other sites. Amazon doesn't provide the only self publishing infrastructure nor are they the only place to post a review.
Sorry, but I think the goal of making the reviews on Amazon more credible is worthy of pursuit.