Because my camera is there and I do not like the pony bottle trying to occupy the same space as my camera rig so I want the pony up under my arm in use. I may also clip the camera arms off to the shoulder D-rings for hands free entry/exit or emergency need for my hands and to prevent the camera from swinging about. A pony hanging down means it has a lot of range of motion, enough to smack me in the chin or teeth during a back roll entry or hit my camera. I like my pony slung up tight as possible and as parallel to my body as I can get it.
The pic #12 is a study on how not to do everything. Poor trim, over weighted, finning to stay at depth, arms crossed but held out for some reason without flexing shoulders or bending elbows, pony hanging down swinging about.
Cool. Very valid reasons. None of those issues apply to me. That is why I see no point (for me) in taking the extra step of adding bungees to tuck everything up tight next to my body. To each their own.
Also, I can't see your pics. They don't show for me for some reason. Not sure if it is just me or if others have that issue as well.