That
used to make sense in the early days of Smartphone era. Precisely because Digital have zero variable cost for each additional goods. And Apple wasn't very straight with those rules. They only enforce it on Software and Games, not services.
Now everything goes through your Smartphone. And the idea, to quote what Apple has been saying in court, they need to recoup those API cost. As they are using their API, they want a cut. Since the Apps for both Physical and Digital goods uses those API, and Digital Goods doesn't necessary use those API for creation. ( e.g I use Windows to create ), why are they only charging Digital Goods and not Physicals?
The only reason why Apple charges 30% of Digital Goods is because they know Digital Goods have zero replication cost. The cost of an additional Digital Goods is essentially zero, they want 30% of it. And Physical goods have basic unit cost. So they are charging base on the product margin. And this was clear in the Wordpess case, once they look at domain name registration where the whole industry is basically operating with 0% margin. Apple decide to put an exemption on it.
Then became a question, how did we arrive at 30% in the first place? If you look at Amazon Web Store, they have different percentage rate for different product? Why? Because they is how the market have worked over the years. They are basing the commission on current market rate / margin. Just like your Super Market has different margin for different product.
Ever since Apple decided on their Doubling Services Revenue by 2020, they have chased down every single 30% services. It is sad how much good faith they have burned.