Sounds like just another bit of hysteria dreamed up by Apple haters
I haven't installed iOS 9 yet, but every smartphone I have ever had has had the tendency to switch to mobile data when out of range of WiFi. Sounds to me like they're simply adding the capability to have fine grained control of that handover, ie that are adding a cost-saving facility that happens to be off by default, for backwards compatibility with earlier versions.
If you don't want that to happen, there has always been a setting to completely disable mobile data, has that changed with iOS 9?
But as I say, I've not yet installed iOS 9, so may be missing something.