The reason why in-app purchases won't be available until Windows Phone 8 might have to do with the Microsoft Wallet Hub. Additionally, MSDN documentation reveals that the Redmond based software giant has informed its developers that they won't be able to add in-app purchasing until handsets supporting Windows Phone 8 are released (see picture below).
In-app purchasing allows users to switch from a free to a paid version of a game, make purchases in a game for extra weapons or turns. For example, with the popular Draw Something game, users can purchase "bombs" for additional words to draw or to help solve a tough word. Users are charged on the same credit card they have set up to purchase apps with. For developers, it gives them a way to up-sell customers and grab more revenue with each app.
source: TheVerge via PCMag