The only thought that comes to mind is if they believe Earls Militia is large enough that they would have to counter assault to push them off whatever land they've claimed on the Tranch, as if this was some flags and zones Battlefield map.
I don't really think its occurred to them that any militia large enough to require an assault to finish off after throwing themselves against your defenses, won't throw themselves against your defenses. They can just wrap around and shoot you from every direction, you'll never stand a chance. They have no hard cover, no real reserves of food or water. The campers and home aren't gonna stop bullets, rifle rounds will go right through that dome and the rest of it.
Hell, if Earl's been learning anything from /k/, they'll just rig up a drone as a firebomb and smash it into the tranch roof. Or just put a few rounds into the solar panels to fuck them up good, kill the power to their freezers and water pump. There's no sign of the tranch having any reasonably deep pantry, just instant foods, they'd be starved out within a day. A couple bored high schoolers could ruin the place, much less a coordinated militia.