Why would it have too be a pornapp in App Store? Apple allows movie players which play any movies of the right data format (mp4, etc). Why not a VR player with data files obtained elsewhere? The game consoles already have VR playing capability for VR formatted data files (e.g. Wipeout on PS4). As long as a book is either pdf or epub it can be trivially read regardless of content. I don't see an issue here.