Middleware company Tapjoy has complained publicly that Apple’s ban of "pay per install" apps and promotions has adversely affected its business and a majority of iOS developers who want to use it. The ban was put in place because of Apple’s concerns that developers were paying Tapjoy to boost their game’s ranking in the list of top 25 apps.
When an app uses this system, a user is incentivized in some way to install another unrelated app. The developer of the unrelated app then shares the revenue with the original app.
Tapjoy argues that incentivized promotions are "commonplace" and allow developers to create an additional revenue stream. The debate may be that Apple felt like it wasn’t getting its fair share of that stream. According to VentureBeat, Tapjoy CEO Mihir Shah attempted to institute a compromise, where a cap "prevented developers from unfairly breaking into the top 25," but it was rejected by Apple.
A Tapjoy survey of developers showed that almost half had begun to receive a large number of customer complaints because of the "sudden inability" to earn in-game currency from installing other apps.
From Apple’s perspective, this is a move to bring back fairness of app rankings.