I think its because its not an airfield/vbase, regardless of the concrete it will be a ditch.
It's not the correct type of "concrete". Until recently, we still could safely land place at the HQ because it had the 'right' kind of concrete, but not since the buildings there had been redesigned.
For GV it's now like any other non-concrete spot on the map, it will only be a ditch if some enemy if you are in friently territory (meaning closest
baseis friendly) and an enemy is still in range. And just like any other place on the map, you can land a GV safely if no enemy is around at all.