Will not happen (and why should it) on Apple devices. Because for any In-App purchase, Apple takes a cut of 30%.
That's the reason why, as far as i remember, the Steam app was changed to remove the ability of purchasing.
I expect that there is a similar rule for the Google Play Store.
As the previous posters said, the Gog Store works well enough on mobile devices to make purchases. I personally cannot understand the "App-Hype", where every effing website wants you to install their app, instead of just building a proper, resonsive design website.