In desperation, I have tried resetting my phone as a brand new device and not restored apps or settings at all and it still uses data across the cellularnetwork whilst on Wifi.
You may face charges from the hotel or airport providing internet service, or from the local cellular company if you roam on its datanetwork -- as most non-residents must.
The feature gets her some connectivity, though users doing bandwidth-heavy tasks like watching video over the cellularnetwork can end up with hefty data charges.
This is a company whose primary business is the provisioning of a global virtual cellularnetwork for primarily corporate clients to offer deeply discounted voice and data GSM services through the use of specially engineered SIM cards and an intricate supporting infrastructure and software for least cost routing and simplified phone number schemes.