Theory: these are compromised accounts being used in a review farm. The hacker has the accounts review a few random apps the account already owns. This is why the fake reviews are primarily located on apps that are popular and cheap - as those are the apps users are most likely to already own.
> This is why the fake reviews are primarily located on apps that are popular and cheap
Popular and cheap are relative terms. The Mac App Store is vastly less popular than the iOS Store. And even the cheapest upfront paid apps are significantly less popular than the infinitely cheaper free to download apps.
