Edge case was perhaps the wrong term. It certainly is not a majority case. As I said- make it an option, everyone is happy.

Not the person who turns on the option to make their life easier and then complains because their kid spent $400.

?! Are you suggesting that everyone should be denied the choice because some people are unable to make informed decisions?

This is confusing authentication and authorization. Is this phone legitimately tied to this Apple ID? Yes. Is the owner of the account authorized to make such a purchase? No.

A short appstore PIN could solve this much more easily.

I'm not so sure having _yet another_ PIN for users to remember would be a good idea. And besides, a short PIN would be far easier to deduce by looking over a person's shoulder.