I think there is a significant difference between a "Like" on FB and product reviews. "Like" on FB is rather generic, relates to the whole company, while product reviews are quite specific and personalized, thus, supposedly, much more influential.
At this point in history (of the Internet, that is), most of us are acutely aware of how the whole system can be rigged by shills and marketing practices of sweepstakes and discounts for Likes and similar. Being aware means that I approach any information contained in the number of Likes, number of stars, individual reviews, etc. with a healthy dose of skepticism.
Having said that, I believe companies are perfectly within their rights to exchange discounts for Likes, but not to pay for 5 star reviews, as mentioned in the article. It is also perfectly ok to discriminate between customers who agree to the terms and those who do not. Unless, of course a new, legally protected species (e.g., obnoxious photo-background buyers) is added to race, gender, national origin, etc.