It actually isn't. The idea is to create a "Buffer" zone between combat areas and spawnrooms, for teleporters, building charge, and adding a bit of run distance.
The constant wave of new troops means you have to push a lot of people through generally narrow, easy-to-defend passages before you can reach the buffer area. How often do you actually get spawncamped there? As compared to, say, Dustbowl 1. where the three exits are spread out, but directly open onto a combat area. (Dustbowl 2 and 3 also use this buffer zone, same as goldrush 2 and 3, (goldrush 3 is even in a height disadvantage!), Mann Manor (a particularly small one, but you see the principle at work), and pretty much any map with a space to build ubers in. They're near enough mandatory on a/d maps to avoid spawncamping from second one.)
All those three spawns you cite are also well-designed so the doors face away from each other- it's awkward to see both at once.
As for good spawns, Upward Blue, for the story, and the feeling of fresh air, all three Thundermountain Blue, for the story and the cool detailing, and Mountainlab Blue for the airboat and the little cove.