It actually makes perfect sense anatomically.
I am 6'4" so I squat to sit on a toilet anyway, but for short people - sitting on a toilet is like sitting in a chair. There is a diaphragm inside of your anus that relaxes when you squat..whch allows for an easier...well...poop. Sitting in a traditional sitting position, the diaphragm is still closed and you must push the excrement and force it open.