I rate in half stars on my site, but I will round up or down on Amazon depending on a gut feeling. 3.5-4 stars means that I would recommend the book and mostly that means that an Amazon rating of 4 is recommended. 4.5-5 means very highly recommended and/or exceptional which will most often turn into an Amazon rating of 5.
But I believe I am possibly a fairly low rating reviewer with only a couple of books a year hitting that 5 star mark. Perhaps I should be surprised that people still actually want me to review given that 5 star reviews on Amazon are such cheap currency.
Back to the original, it's a bit of a shame that this kind of childish behaviour continues once we have theoretically graduated from the playground. But maturity seems to be a very rare commodity in this world so I can't say I'm surprised.