Forgetting about any atmospheric capacity, the Eagles are supposed to launch from a Moonbase to orbit, so a (lunar) ground indicator makes sense. A moonbase landing is the standard return to base, pre-breakaway.Well both the Eagles and Vipers can operating in space and planetary atmosphere so even if not necessary during space flight still need to be in primary flight display.
Also think it would help those onboard to be oriented the right way. I'm sure the craft could fly no problems on it's side, the disorientation of the passengers and crew would be pretty nasty though.