developers can offer other payment options outside the App Store, but they still need to pay Apple a 27% fee. In addition, they have to follow some guidelines to promote an external link while also reporting to Apple about every purchase made through this external link so the company can charge a fee.
I think the meeting at Apple when they were discussing this must have gone something like Well how hard can we fuck them before their arsehole ruptures? A bit more? Sure let's go for it. Lube? Nah we don't need that.