Many people don’t realize it, but Apple’s strong stance on privacy means that as long as you’re using its payment system, third-party developers don’t get to know anything about you by default.
Apple handles all of the transaction processing, and only pays out the total of all monthly or quarterly transactions to developers. It doesn’t give developers a detailed rundown of who made those transactions — not even Apple IDs are provided. They just get the money and some encrypted and anonymized identifiers so that they can tie the subscription to your device.
This has actually been another point of contention that some developers have with Apple, as they also feel that it prevents them from maintaining the customer relationship. While that concern is a fair one, it doesn’t change the fact that by default, when you simply make an in-app purchase, your privacy is completely protected. Developers can’t try and market to you outside of the app to convince you to buy more stuff, or even try to nag you to reinstall a game that you’ve grown tired of. It’s a very consumer-friendly approach.
Of course, apps may still encourage you to sign up for an account with the developer, but this is always under your control, and you know what info you’re providing and what you’re signing up for. It’s also almost never necessary to do this for simple transactions like buying in-game currency, so you’re always free to decide if signing up for an account and giving up some of your privacy is worth the benefits that the developer is offering.