> The yearly subscription costs the same in both cases,
Is this through in-app purchases? It would seem counterintuitive to let Apple take 30% from the yearly subscription if they've gotten away with distributing outside the app store for so long. (but they may have a special revenue share agreement)
That's how it has always been. If a company acquires a new subscription through the App Store then Apple processes the subscription and takes 30% the first year and 15% of each subsequent year. Apple doesn't claim any percentage of subscriptions acquired outside of the App Store.
If Microsoft acquires a new user and subscription through the App Store then Apple is entitled to their cut. If Microsoft acquires a new subscription organically outside of the App Store then Apple is obviously not entitled to a cut.
If Microsoft doesn't want the ability to acquire new subscriptions through the App Store (and pay Apple for it) then that's their prerogative, but right now that's not the case.
If you purchase an in-app subscription through the App Store with your Apple ID, the only option is a yearly subscription.
If you purchase a subscription directly from Microsoft's website, you can purchase either a yearly or monthly subscription.
The yearly subscription costs the same in both cases, and is cheaper unless you only want to use the app for a few months.