Really ? Surely it would have to be unbelievably deep for one to drown wouldn't it or have steeply sloping inwards sides !??
They do have decent muscle tone , clearly not as much as a Boa but I've seen them telescoping around and stretching up whilst yawning . I actually think , as mentioned above , that it's possibly just a case of wasted space as they probably won't spend long soaking like many other types of snake do .