Since Office 2013 includes a cloud service anyway, why not let subscribers get their app for free? The subscription fees are where they're getting their money in that model, not the app sales, and the only way to keep Apple from getting your money is to charge nothing for the app itself. It works fine for Carbonite, CrashPlan, Dropbox, Google Drive...
Oh wait, I know why Microsoft won't do that. Because they've always been more motivated by profit than by making things convenient for their users. Say what you want about Apple, but they are extremely user-focused... except when they're convinced they know better than the user what the user *should* want. Sometimes that leads to great things, and other times it's bloody infuriating when they remove a feature people actually used.