It's all about making money, not whether it's technically possible or not.
Witness O2 and their charging for overseas text messages, notwithstanding that the text messages are borne over a free control stream for the mobile and then sent out of the same internet pipe that's used to deliver SMS to other ('free') networks, just via a different VPN.
Can you tell i'm a bit annoyed my wife's "£35/month all inclusive" package turned out to be a "circa £100 a month because she's foreign"?