They are designed for visual privacy. Some cultures are more private than others about these rituals, but there is still a general theme of privacy. Having to defecate in the face of mild immediate tension isn't uncommon. Anxiety provokes the peristaltic action of the bowel tract - and blatt, the quick anxiety morning crap. But sustained tension can also be responsible for constipation.
Few things are as taboo as the references to and uses of body excretions.
And not even blood and visceral gore are a match for feces when it comes to turning stomachs. Both blood and urine are usually sterile of pathogens - more so than most spring waters. Yet if one had to choose under penalty of death whether to drink blood or urine, people would usually not choose urine. The idea that we have evolved our repugnance of elimination products for reasons of sanitation doesn't hold up entirely, because of the sterile nature of urine. Unlike birds, early mammals saw the world through their noses.