[imtringued]

Most likely according to Guideline 3.1.3(b). The biggest problem with the app is that iOS only users can't actually create an account inside the app. It's perfectly fine to use a windows computer to create a paid account and then use that same account in iOS but since it's not possible to create a paid account on iOS and then use it on a windows pc it's violating the guidelines.

[dmitriid]

Because Apple forbids you from even describing how you can sign up outside Apple. Much less provide a link.