This reminds of me how Three Mobile in Australia drops back to Telstra when roaming. This is by default and the user is none the wiser (unless they had the knowledge to turn off roaming).

I think every person I know who has a Three phone used an amount of data in their first month whilst roaming and had a $100+ excess usage charge (often much higher).

I'm sure it's all spelled out to them upon purchasing, but it's not clear and your average user has no idea what roaming is.