The OFT is concerned that some app makers make their apps available for free or for very low cost in an app store, but then have various systems for making money via expensive in-app purchases. In March a British child hit international tech headlines because he ran up a bill equivalent to about $2,500 when his parents unlocked their iPad so he could make in-app purchases for the game Zombie versus Ninja. It's exactly this sort of issue that the OFT may attempt to prevent--though it seems unlikely to try to ban in-app purchases, it is trying to make sure the industry obeys regulations about deceptive advertising.
The OFT has noted that of the 100 highest-grossing apps on Google Play some 80% were sold for free but made money via in-app systems. Apple agreed to pay out around $100 million in compensation in February to settle a lawsuit brought by five families whose children created huge iTunes bills in this way--doing so before Apple added an extra password access layer to prevent mistaken in-app buys.