This whole default business is a pain in the neck. I think it'd be better if the dialogue that asks for your password for buying apps, media or IAPs had an extra option button:
1. Authorise once
2. Authorise for 15 mins
Then everyone can choose what they want to do as they do it.
Also, once TouchID is deployed across all devices, it can be Authorise Once all the time, because using TouchID isn't close to as much of an annoyance as typing in a password.