A caller on rush yesterday said that Amazon was not allowing any ratings below four stars. Im not sure if that was ever confirmed.
I used to occasionally review products I didn't purchase from amazon but locally, usually very positive but maybe a few somewhat negative, and they were published after a short lapse. The last time I tried to review an item and not be a verified purchaser, I was blocked. That was within the past year or so.
I think people can still comment under a particular review; some got quite heated and lengthy in the past. I know the book I commented on I had not purchased and didn't intend to. It seems to happen very rarely now, if at all.
I didn't want to risk my generally favorable, I think, reputation by playing with the stars and faking a review as I have no desire to purchase the book. There are usually enough details getting out that it's not worth reading the entire book, don't like kindle and don't read but a very few books any more.