Do you not get it? People are used to connecting to wifi at home meaning they're using wifi NOT Mobile data. Its a simple concept that works well for people avoiding high data chargers.
People know that if they're using apps with mobile data while out, that downloads will count against their mobile data - its a learnt behaviour. Its common sense. If you're not connected to wifi, then anything will download over mobile data, if you're connected to wifi, everything will download over Wifi. Apple has gone ahead and changed this to, sometimes even though you're connected to your Wifi, things will download over your mobile data without you knowing.
If Apple wants to change things, especially things that could lead to high external costs, then they need to inform users properly with warnings, and making it an opt in system.
What spoilt, because I disapprove of Apple screwing people over? Its fairly well learnt that if you're NOT connected to wifi, that your mobile data will kick in. People know that, just it used to be that if a device were connected to wifi, it would use that - now even if you ARE connected to Wifi, your device can still use mobile data without the person knowing.
It should be off and an opt in feature.