I have written some for Tesco, they are on their website. I only do so for what I consider a five star product. I have found it quite relaxing to do, as I enjoy writing, so I tend to make the review "a good read".
I have had email requests to write reviews from several companies from which I have bought something delivered by post or courier.
I would be quite happy to do so, but I always read the Terms and Conditions.
One, to my amazement, had outsourced the review system to a foreign company and the terms included that if anyone complained about my review then I would indemnify (a long list of companies and their staff, affiliates, subcontractors etc) for all the costs of doing so, "including reasonable attorney's fees".
I decided not to write a review for them because of that. 