Costs of development is covered by increase in profit from sells of real-life boxes with descent expansions. That's why app was made free and big campaign monetisation method was changed.Because development costs money. That's all.
You just can't expect a company to give you something (even digitally) for nothing.
So it's perfectly OK to give content for "free" if you know how you will monetize your generosity
Minus the cost of printing, miniatures, packing, shipping, storing, and distributing the games. That stuff still costs money, leaving a small portion left to subsidize app development.
Lots of people bought the core set because of the app, but that probably wasn't enough to cover the development cost. Not to mention that they're not going to sell a box game for every release on the app. People already have it. They need to cover the cost of developing new digital content (all the costs or writing, illustrating, editing, and graphic design, plus the cost of coding and QA) by selling it. There's no way it could be completely free.