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.