It's possible to discriminate against foreigners, for example, by requiring presentation of a type of national ID that they can't easily get.
Not come across that anywhere and I can't imagine it surviving a challenge.
Existing rules already make unbundling when roaming possible. So there should soon be third parties offering deals to the minority who have "above average" requirements: single telephone number but calls and data can be handled by other partners when travelling.
The telco's are eking out charges as long as possible but the increasing ubiquity (I'm not sure if that's an oxymoron) of wireless when travelling is really eating into their margins. Worth noting that this generally affects the operators in holiday countries more than the travellers.