Apple briefly pulls hit iOS game 'Monster Strike' for bypassing in-app purchase system
Monday, August 31, 2015 at 8:21PM 
For a short time over the weekend, Apple pulled the Japanese-developed Monster Strike game from its App Store after they found out Mixi, the game's developer, was trying to get around Apple's in-app purchase system. The game was down from 5:30 pm on the Saturday of August 29 and went back up around 1 am on Sunday. It's said the app may have lost around US$600,000 in revenue.
It was said Apple found a "serial code input form" inside the game that gamers can input into the app to get currency or special items. These can be bought on websites or through referrals. But this bypasses Apple's requirement that you can't sell anything outside of its in-app system, because basically you're cutting them out of profit and the 30 percent revenue they get from all activity within the App Store. The move can be seen as a stern warning from Apple to not bypass its rules.
Source: Cult of Mac