My sense is that airlines traditionally believed that departing RT passengers would be willing to pay more (or have less price elastic demand) for a flight operated by one of that country's airlines. Even though they are perfectly fluent in English, Holland residents prefer that FAs speak Dutch and do Netherlands-style service, while USA residents prefer FAs who speak American and interact in the American style. I'm not saying it is/was true and JVs seen to limit the premia. OTOH, Americans, for example, might prefer to start a trip to Japan on a Japanese airline with a genuine Japanese meal, etc.