> I really don't see how Apple would successfully bill for their 30% cut
Very simple. Apple adds such language to the developer agreement. "We get a 30% cut of revenue for any services used on iOS devices." Apple requires companies to submit some form of user-level payment data. E.g. "iOS device XYZ paid via our website, we owe you 30%" "iOS device XYZ paid via IAP, we don't owe you" etc etc etc etc down the line.
Apple also adds provisions where they can audit your books. Apple adds provisions whereby being off by more than X% results in penalties. Apple adds provisions whereby they can request an independent audit of the companies books (quite reasonable with a "you're honest, we paid, you lied, you pay" provision). If all else fails Apple obviously retains the right to sue to enforce and a court can require a company to provide apple financials as part of discovery.
It's really not HARD per-se. I imagine the only reason apple doesn't do this as that it's just so much complicated than the IAP route.
Probably the thorniest issue here is that it would create some perverse incentives. Netflix might chose to remove their app from the app store in favor of telling you to just go to the website again avoiding the 30% cut.
Furthermore, what would entitle apple to their 30%?
I just bought and installed a card control badge access system at our office. I downloaded/installed the app on my phone. Should apple get 30%? If so, 30% of what? 30% of the total system cost including install? 30% of what I paid for the hardware? 30% of just the controller hardware cost? 30% of the annual support maintenance cost? I don't even KNOW most of those costs because they aren't even enumerated to me. I just paid a big lump sump. Super messy.