I don't think most people who aren't Vine reviewers are happy with how it works ('cause they get free stuff). I have seen Vine reviewers leave bad reviews, and I know because I've been yelled at over it.
My impression is: you pay, you get 5 stars. They buy a product themselves and didn't like it, you get 1 star.
Maybe it's just not an issue for most people because they don't know that reviews could be bought, so any and all reviews are great for making the sale from Amazon's perspective. If the minority that chooses to shop elsewhere grows too large, they can always make a dramatic gesture, punish a dozen people with fake reviews and donate to a children's hospital.