The guys making the planes and the guys making the terrain are different groups AFAIK. So the terrain will get finished on a different time frame from any of the planes. Generally the planes will be first to release because they take less time and about half way through the planes the terrain will get finished and put up for testing. This is just based on previous games. BoM and BoK were like that. the Terrain for BoS was release with the first plane though since you couldnt test the plane without a place to fly
The way it works is more like RoF than most other games. BoS, BoM, BoK and BoBP are all the same game with different content. They all install into the same game folder. So if you preorder BoBP you will have access to the BoS, BoM, BoK terrains. So even if you only get the BP game you can still test the planes on the old terrains. (unless they changed it recently?)