I have always thought the division is to basically protect Apple's cut of the revenue for "App Sales". Not, strictly speaking, because they want to take 30% of every possible purchase or service -- though I am sure they would be happy with that and do desire that to some extent. However more and more every year sales of an "app" become smaller and smaller and everything is a subscription - so it seems this thought is rapidly if not already outdated.

The world we now live in is that all "apps and programs" can be either a one-off purchase like a physical good or a subscription. That subscription might be for the "app" or it may be packaged as a subscription to a "service" that the app is facilitating (the line between the two is very blurry). Additionally, all digital goods like books, music and movies may also be a one-off purchase or a "subscription" like audible, apple music, etc.

The line between all of those is blurry such that, any set of rules you write leaves it open to convert "app sales" to "services/subscriptions" or even to bundle access to your app or it's content as "digital goods" to avoid that 30% cut. People would (and do) switch their sales model to then avoid that cut.

I don't have a solution and I don't think Apple or the open webs way is specifically better in every aspect, but I do think it's interesting to think about the problem space.