It's not a root account issue, it's that they have already bought all the services there are to buy. One would assume that once you buy a service, they would stop prompting you to buy it.

Though, in my experience, that isn't true with Apple. Even when I tried Apple One, I still saw bugs in the experience that led to ads for things I already subscribed to... particularly in News+ and AppleTV+.

No, hardening has nothing to do with it. They have a feature flag system deeply embedded in their software. And such systems can totally distribute different flags to different people. And that's how you could easily create VIP accounts with different behaviors than the rest of the world, like all ad festures turned off.