Kind of like paying cash instead of card at a restaurant.
Edit: Interesting replies. But selling physical goods is an exception to the Apple tax. Maybe they can sell you a magic coin for $1 and ship it to you and that magic coin also coincidentally authorizes you to use the app...
I expect that's not allowed either. Look at Audible. There is nothing that tells you that you have to sign into the website to buy books, because Apple won't allow it, and Apple will take a 30% cut if Audible does it in-app. They rely entirely on users having prior knowledge that (a) you can buy books at all, and (b) the app is just a player, and the store is on the website.
"Apps and their metadata may not include buttons, external links, or other calls to action that direct customers to purchasing mechanisms other than in-app purchase."
[1] https://developer.apple.com/app-store/review/guidelines/#pay...
You install HEY, sign up for a free trial, and the first email you receive is an ad to sign up on their site for a discount.
If you sign up with the app, it’s 30% more.
Edit: Oops, my recollection is almost 10 years out of date[1].
[1] https://appleinsider.com/articles/11/06/09/apple_backs_down_...