I have seen it, contractually, where 3rd party brands that (say) Target is advertising, knows that, and agrees to, Target advertise their product in multiple channels across a certain timeframe. This removes the need for said 3rd party to "approve" every potential ad target places and allows for a more efficient advertising cadence. That is certainly not the only way those relationship function. Regardless, I'm still not seeing where Apple is behaving any differently than other house-of-brand retailers. What is potentially different here, is that Apple typically isn't seen as a house-of-brands retailer, in the consumers' minds. But it is definitely a measurable part of their business, obviously. I'm open to the idea that I am misunderstanding the claims the article is making. Wouldn't be the first time I misunderstood something. ?Generally, the company for whatever product is being advertised is required to sign off on the ad before it runs. This also allows them to plan their media spending accordingly, so that they're not directly bidding against a partner for ad placement.
The allegation that Apple is running these ads secretly is the really shady part of this. Apple is driving up the cost of user acquisition for the developer, and then taking a cut where they might not have before.
Whether it's technically illegal or not I can't say, but it's certainly not the norm for these sort of things.