Excellent catch, they really mention Apple, Touch ID, and point to Apple URLs for support. Maybe they thought no one reads these past the first paragraph.
Hi I’m the creator of Big Mail. As the subscription is sold and managed by Apple we’re bound by the App Store terms and conditions (things like refunds, liability, etc)
Presumably because the app is being sold through the iOS App Store and in that case Apple manages the relationship with the end user rather than the app developer.