Some Theory 11 apps would be cool. Dan and Dave released app versions of The Red Mirror, Papercuts and The Aviv Project, all of which I own. I can see Foundations working well in app form. They could also take all of Bedros' releases and sell them individually through a dedicated app. Cardistry releases like Solo, Genesis and Monument would be awesome as well. I'd love to see some Theory 11 products on the Appstore, that'd be sweet.
iTunes takes a significant percentage of sales. Far less generous than most platforms. Same with apps on iOS, Apple takes something crazy like 30% of the profits, which is why most apps don't do transactions. After taxes, it's not as fruitful, especially when they can do it through their own platform. I'm sure my numbers are off, but that's the general reasoning. Outside of sales, then apps would make sense.