It looks like Office 365 subscriptions are available as an In-App Purchase (seen on the sidebar here : https://itunes.apple.com/us/app/microsoft-excel-for-ipad/id5... ), so Apple get their cut of the revenue and Microsoft are above-the-board in terms of App Store regulations.
Naturally if you buy a subscription from outside the app, Apple don't get a cut - that's how everything works and as long as you don't give the user a way to get to the non-Apple subscription payment system from inside the app, Apple is fine with that.
Sidebar: Apple does not allow you to provide a link to a website where you can market or upgrade a SaaS app that does not use the app store subscription model. Does anyone have any experience with denoting features which are locked to paid users without running afoul of this rule?
Naturally if you buy a subscription from outside the app, Apple don't get a cut - that's how everything works and as long as you don't give the user a way to get to the non-Apple subscription payment system from inside the app, Apple is fine with that.