I hope you are aware that it is not just EE, and the 50GB is quite generous compared to many other networks. 50GB in the USA for example is a very healthy allowance to get from a roaming perspective with no speed caps or aggressive traffic management (at least to my knowledge). This type of policy is adopted in many countries.
Generally it is to prevent abuse by obtaining a SIM from a different country and using it continuously abroad. Roaming customers can put a strain on the network resources in the countries they're visiting and this has to be managed so that residents continue to have satisfactory bandwidth as they are the core customers.
Operators have to maintain relationships and contracts to allow customers to roam on foreign networks. Sometimes the conditions to offer roaming in a country are dependent on a Fair Usage Policy being in place for roaming customers or some other form of traffic management for their customers abroad.
Obviously the networks have no way of charging customers roaming from different countries directly, so the funds from roaming charges and surcharges are often used to support their own network costs that are incurred as a result of customers visiting from abroad. The same goes for other nations, where the fees they charge their own customers are used to support the network running costs associated with customers visiting from abroad.
