But when your app is 1.99USD and somebody in Australia buys it for 2.49AUD based on Apple's baseline price tier conversions, Apple still pays the Australian sales tax and GST due on that. Apple administer the international business and tax obligations for doing business in every jurisdiction. When the AUD falls by 25% against the USD because of a slump in domestic mining exports, Apple just absorb that cost.
Global digital sales aren't just a matter of processing money; and wouldn't be even _if_ credit card networks and digital banking had the coverage they claim. There are significant legal and business overheads and they all get subsumed by that 30% cut.
Is that profitable for Apple? Yes, but largely because they can leverage the legal and business synergies they were already committed to for their physical presence in so many countries, and the global data processing capacity they already need for their integrated online platforms.