I originally thought that Expeditionary Force was denied to the starting Regions in order to disallow easy outpost teleports (included with joining EF) to these areas. But now I'm wondering if you can sign up for EF for a region you've never visited before...

Anyway, according to the game's core storyline, the nations shouldn't have any more claim to "their front yard" than anybody else. They're supposed to (still) be recovering from the Crystal War, unable to exert any real influence at all beyond what the adventurers do for them. Witness Gideus and Ghelsba Outpost.