Nope, there's a cottage industry of review-for-discount sites that give people discounts/free items in exchange for reviews, but wink wink don't feel obligated to give it 5 stars.
Back when I was among the Top Amazon Reviewers, I'd regularly be asked to review products and (especially) books (primarily those that were self published). I rarely said yes, but when I did I treated them exactly the same as I would for a product I'd bought myself. That included 1- and 2-star reviews... though usually I tried to weed out the crap before saying Yes to such an offer.
The cost really never influenced me. I can afford a $10 novel that I'm interested in. So if you give me that book for free, it's no big deal.
In point of fact I learned to say No to most such offers. Usually novels are self-published only when the author cannot find a traditional publisher. And the traditional publishers usually refuse the book for a good reason. I don't want to waste my time reading crap.... not when there are so many good books to read instead.
Also, the self-published authors really didn't like it when I gave their books less than 5 stars, and magically the review would be downvoted -- as well as the last three unrelated items I'd reviewed. Presumably those neginators were from the author's friends and family. So I just said No, and moved on to books from authors I was sure I'd like.
Sure, some authors build a fan base by themselves, go viral, and a publisher gets interested in them (see The Martian, or 50 Shades of Grey, etc). But as any viral phenomenon, quality is not necessarily the reason (see 50 Shades of Grey).
What if you're a good author but not a good marketer/salesman, or just not interested in that part?
I'm biased in all this. I have a full time job, I write as a hobby, publishers ignored my queries, so I self-published my novel. It's far from a masterpiece, but according to a majority of people who have read it, it's also far from crap. My main problem is how to get it in the hands of more people, but I don't have the time or the energy (nor the inclination, really) to also do marketing & sales. That's what publishers are supposed to do :(
I design, import, and sell products on Amazon, and when launching a new product I offer them for free or at a deeply discounted price to people who agree to provide an honest review. From my experience, across hundreds of transactions, I can say that reviews that are solicited with free or discounted products are much tougher than those received organically. Most people participating in these review groups are so concerned about being viewed as biased that they move far, far in the other direction. If your product can earn decent ratings with incentivized reviews, you'll do very well with organic reviews.
Amazon offers its own product-to-review system called Vine that is invite-only. Not all sellers can participate in the Vine program as it's available to Vendor Central sellers, meaning that Amazon purchases the products directly and lists them as 'ships from and sold by Amazon'.
https://www.amazon.com/gp/vine/help
Additionally, on each review, the reviewer is required to explain that they received the product in exchange for a review.
How would you recommend getting reviews for a product, apart from offering them at a discount?
Edit: Added link to Vine FAQ
Better yet, don't review free products. It's just spam that the rest of us don't want to sort through, and it buries the truly unbiased reviews.
I have heard of organizations like this: http://revleap.me/ (I'm sure these guys are completely innocent, they just happened to come up in a DDG search for "get positive amazon reviews".)
If you search for "work from home" you will find lots of shady organizations paying people (pennies) to post reviews, for example on Google Maps.
It's not a huge leap to imagine a service which pays random online workers to contact negative reviewers with sob stories begging for the review to be removed. And it's not hard to imagine that a disposable worker such as that would indeed be fired (simply not offered additional work) if s/he fails to get a review removed.
The organization I linked too shows a picture of what appears to be some kind of negative review alert delivered to a smart phone.
I ended up publishing a glowing review of the CodeBug anyway (https://www.stavros.io/posts/codebug-review/), but that's because I sincerely enjoyed it. I didn't get around to doing anything with the BeagleBone, sadly. Maybe that's why they didn't send me anything else, though :P