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I like walled gardens. They are safe and, for the most part, keep out the predators. However, when one sneaks over the wall, the results can be ugly, to say the least. With the iTunes App Store, one of the key supposed advantages for end-users is that it is a walled garden, and Apple is providing a safe, secure environment you can trust in. Apple vets the apps that are approved and provides a mechanism that allows users to review apps. The company even set up the review process so that only those who download an app can review it.
Your comment about the "C-shape" as somehow caused by fake reviews is wrong.
It can be caused by users downloading free apps and immediately deleting them while leaving a one star review (probably because the app was not what they expected).
Check out our app, Arithfit (It's for having fun with maths)
We made it free for one week to celebrate the release of the iPad, and that's when it accumulated all the one-star reviews.
The other 5 star reviews are all genuine, and have been growing with time. The reason is that people who actually download the app because they are interested in what it does do like it a lot in the end. While people who downloaded it during that short period when it was free weren't really interested and just left a one-star review when deleting the app. In our case, 20 one-star reviews out of many thousands who downloaded the app for free are nothing compared to 10 five-star reviews out of the few hundred users who actually purchased and used the app.