The emails can definitely be a good thing for reminding someone about billing, but they're not entirely effective. Most people would ignore emails such as these or they would read them and forget the information as something else is more important, or just rarely check their emails (like weekly, fortnightly, etc).
So the default behaviour matters most in this situation, as evidenced by opt-in vs opt-out for things such as organ donation. When it takes action to opt in most people don't do it (even though it's a good thing), and when it's opt out then most people don't take the actions of removing themselves from being an organ donor.
In this case with all the emails that Apple sends having the default behaviour be opt-in would actually still work. People that want to continue using the application should take the necessary steps to subscribe. If not then they either didn't want the subscription, don't use the application, or they didn't read the messages and are relieved that they're not spending money unnecessarily.
Therefore having opt-out be the default for subscriptions means that Apple is doing it explicitly because it makes them more money than the reverse situation. It is actually user hostile but with the thin veneer (typical of T&C or Privacy Policy) that they "told you so".