AFAIK nope, not illegal. Apple still would have the ability to create a rule and system that generates a commission on transactions from purchases on mobile Web (ios) but that doesn't mean this implementation is illegal, it just means Apple needs to implement that system and rule.
Also remember it's probably more of a pain to chase down the payments post the transaction.
Even if Apple does implement a rule like that, Apple will hate it if they have to go after third party companies for the money. With embedded iOS stuff, it's all automated with an Apple cut on the backend.
This is no different than other industries.
In the retailer/supplier world, each side wants to control when to pay the other guy.
The retailer wants costs and deals automated so it automatically comes off the order. The supplier prefers to charge retailers full price and any special prices or deals are paid later via the the retailer sending in claims saying the supplier owes deal money.
The power is in the hands of which side wins because it makes it a tug of war from the other side to get their due. All the while, the winner holds onto the money as long as possible and hopes the other side doesnt notice or have the man power to track things properly and ask for payback owed. Or they drag it out and legit pay claims..... but like 8 months later.
In simplified terms it would be like selling a pack of Oreos.
- The store and Nabisco agree on $2.00 That's the deal. The regular price is $2.50
- Nabisco prefers to charge the retailer $2.50. And come back to them later for 50 cent claims
- Store says fuck that. Just bill me $2.00 upfront. Dont want to get into admin hassles later and missing out on 50 cent claims
If Nabsico wins and sells for $2.50, they have nothing to lose. At worst, they pay back the store all the claims x 50 cents each. But best situation is the store doesn't get around to it or sends in claims for less than they bought. Even when Nabisco agrees, fine. They'll pay back the store next year when they get around to it (wink wink).
If the store wins getting upfront $2.00, it sounds like a great easy process for all. But what can happen is the store "magically" underpays bills by assuming it's a 70 cent discount. Now it's up to Nabisco to have enough manpower to track this and go after the store for 20 cents per unit they underpaid. It's hard for suppliers to track this because when you got a lot of deals plugged into upfront pricing, it's hard to know if a 50 cent deal or a 70 deal is legit since every store deal is different and it takes a lot of admin work to know what upfront price is legit or not. But if all stores buy at regular price first ($2.50), it's easy to notice if they are underpaying a bill. The store can drag it on too, and not pay back for ages. And it happens a lot. Funny how you never see a retailer overpay for stuff by accident. It's always trying to slip through an underpayment hoping nobody notices.
So you can see whom ever has control of the process has the upper hand.