This is a perfect example of a line having to be drawn somewhere, but wherever you draw it is never going to be perfect in every case. First of all -- app stores need to make money or at least break even. They cost a lot of money to run. So app stores take a cut of all transactions. If they let developers implement their own payment mechanisms, either in-app or with a single link, then developers will make every app…
>Or a rule that "donations" are allowed? Well then you get "donations" that come with "bonus functionality" and so forth, and a new line has to be drawn.
There really isn't any ambiguity about what constitutes a donation. If you receive something in return, it isn't a donation.
I would argue that Google could easily permit donations of any kind at no detriment to themselves.