I have to ask: Why? You just described amazon's Vine Voice program, where the score of the review is decoupled completely from whether the reviewer gets more free stuff - there's no incentive to post bogus reviews.
Further, having been on the receiving end of a couple of these solicitations myself, they usually took the form of "Hey, we noticed you bought $thing. We'll send you another $thing (or something similar) if you write us a review, good or bad"
Neither of these situations seems sketchy to me, inasmuch as I can't see how someone getting a product for review purposes is likely to taint their review.
Further, having been on the receiving end of a couple of these solicitations myself, they usually took the form of "Hey, we noticed you bought $thing. We'll send you another $thing (or something similar) if you write us a review, good or bad"
Neither of these situations seems sketchy to me, inasmuch as I can't see how someone getting a product for review purposes is likely to taint their review.