The problem is not AppStore-downloaded app using AppStore for IAP, it is that I am not able to download anything I would deem necessary on my very own general purpose device if Apple deems it not a good fit for their platform. For all I care force IAPs to go through their servers for 30%, the majority of the users will still use it just fine.

Just put a scary looking, click 35 times on this hidden menu option to allow proper sideloading, not this 3 apps for 7 days bullshit.