yes, as you've now no doubt realised, you could have titled this thread a little more carefully.
The reason it happens is because punters don't want to run the risk of receiving recriminatory feedback if they post negative comments on aw. By the same token, the more postive they are in their feedback, the more positive the feedback they are likely to receive, and, often, selfish motives dominate. It's more important to maintain and add to your own positive reviews than warn other punters about issues or shortcomings in service.