Thats definitely a large problem, larger even than it was with print magazines (where some forgot to separate the ads business from the technical/review crew).
Yes, it would be helpful to have some reviews that are not so positive to be able to calibrate for the preferences of the reviewer (its easier to derive my personal offset from someone else's experience when I know both likes and dislikes), but I fully understand few will publish scathing reviews of things they really dislike -- a waste of time in the end.
Going back to Tyll, my gold standard reviewer, there's a good example: he did not like the K812 and therefore did not want to spend the time to write a review. Many K812 fans kept asking though, so he finally write a somewhat negative review while still keeping things professional. I can understand he was hesitant to spend that time, yet the result was still helpful.
HE also once did a review of some Ultrasone model (Edition 12 or something like that) with that rather famous scene in the accompanying video where he pulls it off in disgust. As I remember it was after that when he decided not to do/publish negative reviews anymore.
So yes, I understand your point. I also understand the impression that review must invariable be paid for. What I want to say is that there might be other reasons as well. It's not that all reviewers are dishonest (though many might not be self aware enough to see their own biases -- which, I'm ready to admit, is hard), but circumstances result in positive reviews. One reason I tried to outline above, others might exist as well.
I also tend to value private opinions higher, derived from gear they bought on their own budget. Yet that introduces another bias: self confirmation for the purchase. The world's never white/black, but always shades of gray... and no, I'm not talking about the book of the same name.
and doesn't mince his words
.... all's forgiven. 
