Because I'm not going to leave a review, don't really leave many reviews. But I'm especially not going to leave a review before I've received service. But if I don't leave a review, I'd be concerned I would be getting deliberately poor service.
And if I'm going to get bad service, why should I subject myself to that?
If anything, I would leave then give a 1 star review saying they give discounts to people who give good reviews beforehand and the explanation I gave above.