Good point about the base set monsters- it's consistent with FFG's policy about expansions being designed to work with the base game independent of any other expansions.
Also, Charmy, you're right that Splig is generally accompanied by goblins. However, just like "Splig's revenge," if FFG wanted Splig's plot deck to be about goblins, they could have done it much more easily than by trying to pick between wilderness/building, and just written "a monster group with 'goblin' in its name." However, that's not the route they went. Those who are displeased with that decision are of course free to houserule.