Shelf space, physical distribution, and store operations, etc costs money -- but so does bandwidth, security, tech ops, etc of all of these platforms.
Why isn't it ok for a digital store to require a markup to sell?
Best Buy/CompUSA/Office Depot employees do not scare the customers and block the customers coming to you directly.
These are the few problems of monopoly that Apple so conveniently takes advantage of.
How does Apple scare customers? By giving them a warning about sideloading?
A warning hardly seems monopolistic? Unless I'm misunderstanding your post
The scare part refers to warning dialogs that iOS would pop up if you used a link to an external web browser to collect payments. It would warn you that Apple wasn't running the payments and use scary language warning you about potential fraud, etc. to try to scare people away. They would also demand a 27% fee from developers for collecting money on the web outside apps if users were paying from following that link.
To top it, when the user actually sees that hidden 'link' and clicks on that, the user is put up with a big screen of message that the external website doesn't have security, privacy etc.
Check it yourself in this article by a developer: https://www.macstories.net/news/an-app-store-first-delta-add...
Another issue with the analogy is that when you buy a PC in a physical store, that store does not continue taking a cut of all software bought on that PC using i.e. unrelated digital stores (but that's precisely what Apple is asking from competing stores in the EU).
But do they cost the same per unit sold?
> Why isn't it ok for a digital store to require a markup to sell?
It is ok, just not 30%.