It is antithetical for apple to create a for-profit store which allows for all avenues for profit to be skipped.
Apple's take is typically one of a referral/marketing relationship, such as how they charge different rates for acquiring a new customer subscription (30% for the first year) vs maintaining a customer subscription (15% for additional years) or having a customer come in through another subscription channel (0%).
To prevent abuse (from Apple's perspective), they mandate that the in-app subscription and purchasing be available and 'encouraged' (as in, not actively discouraged through referring to any price difference from other channels as a 'tax', or linking to other channels rather than offering in-app options).
A lot of the recent news (such as the Hey email app fiasco) unfortunately come from developers trying to interpolate what is acceptable through other examples in the App Store, rather than reading the terms they have with Apple. IMHO different handling of different apps is actually due to the various business models producing a very 'jaggy' line around in-app purchasing and subscription requirements. For instance, Hey decided rather than supporting in-app subscriptions as an option, they would provide a 'free' service tier - but this does mean their app is restricted from pushing the user to a paid subscription tier, including indirectly via links to the hey.com website.
Developers often look at very popular apps which have the ability to publish with no functionality without an out-of-app account with external purchases and/or subscriptions. However, these apps are actually limited to a few types of products, so called 'reader' apps where presumably the user equates the products with consumption of already purchased/rented physical goods (music, movies, television episodes, books, magazines, etc). Presumably this exemption both came from the popularity of large apps published before in-app purchasing was added to the system, that Apple themselves had spent a lot of time implying their various media stores were for "purchasing" media just like in the physical world, and that limitations that would effectively prohibit the use of systems such as Kindle and Amazon Music would create antitrust issues.
Apple obviously is not willing to voluntarily give up any territory along this line of exemptions, which is why other services (such as email accounts or blog hosting) are not considered part of this "reader" exemption. Also obviously, it is also really unfortunate that this complicates what business models are acceptable when publishing on the App Store.