I'm very willing to believe that this move was driven by actual user research. As a user, the last thing I want to do is create a user name and password for your app, click a link to validate my email, then enter my password again into some sort of cross platform widget that doesn't support keychain autofill. Unless it's an essential service like a bank or an airline, I'll probably opt out of using it.
I'm also very lazy when it comes to payment methods. Trying to order food and the app doesn't support Apple Pay? Delete it and do something else.
Clearly there are issues with the entrenchment of Apple at the center of all this, and these problems would be better solved with open standards, but the consistency and convenience makes an actual measurable benefit in the end user's daily life.
