To be fair, this is exactly how I use Netflix. I have a free app that I pay to use completely outside the app store. Not sure of the actual rules, but it's definitely possible.
Apple takes these matters pretty seriously, it is their bottom line after all. I once had an app get rejected by App Review because it was possible to buy a subscription from the WebView that opened the Forgot Password page.
But they will shut down any out-of-band payment options. There is nothing even remotely fine about this.