In the EU example, phones in the UK are marketed for domestic use primarily. This is probably because it is a group of islands and the borders are relatively non-porous.
it makes economic sense to give the best tariffs for domestic use as a result. If that means scalping from roaming rates to discount further, I suspect they will do so.
Ofc, you can switch tariffs, buy bolt-ons to discount international use etc too if needed. Being based in Northern Ireland means that I usually get cheap roaming in the Republic of Ireland thrown in for example.
If you legislate for everyone to get cheap roaming, then I can pretty much guarantee that domestic tariffs will increase.
There is no free lunch. Not everything is a conspiracy either.