I'm sure people do it, the bribes are probably not very good. Sadly the company can probably get Amazon to take down the review in that case.

Reviews for sellers themselves are a joke too, I was looking at some earlier today, via black friday deals. The number of reviews with a line through them and "Amazon takes responsibility for this fulfillment-related experience" was hilarious, when those reviews were things like "the motor doesn't work". I guess those then aren't included in the "90% positive" statistic.