This is not true, in my experience. For example, the EU roaming charges regulations for mobile data allows service providers to apply a lower cap to the data if

This is not true, in my experience. For example, the EU roaming charges regulations for mobile data allows service providers to apply a lower cap to the data if the home tariff is cheaper than €2.25/GB; but almost no providers actually do this, and they instead give the user the full data allowance, at least for providers based in the UK. Before the regulation came into effect, roaming rates were much higher for almost every provider, and an EU tourist would have to buy a local SIM card (or just accept the roaming charges).