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.
People cheer when government makes a rule like this but there is a huge costly enforcement mechanism that goes with this. That has to be implemented and maintained. Making the rule is step one, and there are hundreds more steps that have to happen any number of times, forever. Good luck. Making laws that cannot be enforced just increases the cost of government without having the intended effect. I can't think of anything that the prime offenders would like more than that.
It's a convoluted problem with no good answer so far.