Which, if I'm reading *Playing at the World* correctly, has a lot to do with a quirk of geography: EGG wanted Arneson to show off Blackmoor to the Lake Geneva crowd, but only Arneson (not the whole Twin Cities group) could make the trip.
So DA goes up and has to referee *and* play all the antagonistic roles "against" the Lake Genevans; in the Twin Cities he'd solely been refereeing (a la wargaming norms), while players played both the "adventurers" and "big baddies" in the dungeons.
Thus was born the GM's dual role of referee and antagonist.