I think their argument is that they're providing you exposure through the App Store, so they deserve a referral fee for maintaining the App Store, reviewing apps, etc. Normally, user acquisition (getting a user to even think about installing your app the first time) is much more expensive than maintaining a subscription -- also why so many companies are pushing subscription models -- so Apple probably considers it to be worth more. There's also a higher risk of fraudulent purchases or refunds (and associated support costs) for the initial purchase than a subscription.
In the past, developers were prohibited from telling users that they could sign up cheaper on your website, allegedly to avoid user confusion; I think it was relaxed somewhat so I don't know if it's still the case.
Of course the main reason why they charge 30% for the first year is because they can, and the reason they charge 15% after that is because app/content developers have been starting to gripe about it (probably after realizing they need to make a profit and can't live entirely off VC money). It has little to no relation to how much it actually costs Apple. There's no justification for why it takes a year to get the reduced rate, instead of something like 2 months.
