iOS app store is rich of content and imitation apps as well, it reached a whooping 6,00,000 apps since launch in 2008. Apple always is fighting to maintain rich content in app store, getting a app verified itself is a strict and controlled process barring some exceptions. It is always possible that developers adopt to unhealthy ways to promote their app to the top.
Making sure that a app reaches the top 25 apps list on app store will certainly ensure a huge pay check to the developer. Apple has made a payment of $4 billion to developers till date which accounts for app store purchases only, apart from that developers earn from ads as well. So its quite a motivating factor to get a app into top 25 of app store listing.
A developer on has recently discovered a huge fraud mechanism which he posted on TouchArcade, it seems that a complicated network of bots are being used to push the number of downloads that happen for a app, this ensures that the app reaches top 25 apps list and a better invisibility. From there onwards the app will have real human users downloading and using their app. The person behind this complicated network was charging a mere $5000 which in turn is affordable compared to some genuine rank boosting techniques.
Apple eventually had to ban all apps and their developers promoting apps by using such techniques and issued a strict warning against adoption of such techniques.