Absolutely, but it makes Vine-reviews useless to me. I'd take the product for $0 and be happy, but they aren't offering it to me for $0, so some review that's essentially "it was free, it's fantastic for something that was free" isn't relevant.

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.