My guess is that Apple is trying to reduce the load on their servers from the downloading of apps that a high percentage of customers end up not using. If a high percentage of downloads for an app are being deleted, the people deleting that app will strongly tend to rate it down, which should eventually lead to less downloads for that app from people who just want to "kick-the-tires", which will reduce the load on the App store infrastructure.
This will also tend to reduce the amount of stars, and thus the amount of downloads, for those types of free apps, and $0.99 apps, that people might download on impulse, but quickly throw away. Higher priced apps are less likely to be downloaded on impulse, so will be more immune to this down-rating effect.
If a person really needs an app and is interested enough to look carefully, they can find out how useful an app is by reading the actual reviews that are top rated, not just looking at the diluted stars.
.