Apple's app-store policies are anti-competitive. Even though they recently 'allowed' (How kind of them) publishers to create a link to an external form of payment, where the offer may be better, Apple still demands a significant slice of commission.
I think many users would continue to use in-app purchases as it is a convenient way to pay, but the actions from Apple are poor and heavily restrictive. To save yourself some money, if subscribing, always go to the website, you'll get a better offer than within the app and more of your money will go to the publisher.