Vodafone looked at the idea of abandoning roaming across Europe in the mid 00's and were basically told by the EU it was anti-competitive to do so.
I know that both Vodafone NZ and AU have also toyed with the idea and once again were essentially told in an unofficial capacity by regulators that doing so would raise anti-competition issues.
It suits regulators to force things when they feel like it. Often when companies want to do the same thing regulators see it as an anti-competitive tool.
Vodafone NZ and AU dumping roaming between NZ and AU would be a fantastic move, but what happens when competitors are unable to offer the same service? Many people would say that's just bad luck, but regulators don't see things that way.
weird isn't it. I can Skype or WhatsApp anywhere in the world, so long as the other person has the same software, for free. But regulators say phone companies can't do it Telecommunications..... It's complicated