Neither is fluoride, mercury, aluminum, formaldehyde, aborted fetuses, diseases (alive or dead), arsenic, white sugar, processed flour, et al. The soap isn't chewed. It causes a nasty taste in the mouth, it creates a lot of saliva, and it is spit out in the sink.
The way I saw it, it has been done that way for many generations. Many children find ketchup spicy, they have delicate palates ... I couldn't imagine putting tabasco on my own tongue much less a child's. But to each their own. We all have our own approaches. When they were little I used soap. When they got older and needed a reminder I used vinegar.