It was probably a health decision. Clearly, it's navel is buried. That means it would be impossible to heal a piercing there. Piercings need to breathe to heal. They need to be cleaned routinely, because they do build up lymph discharge as they heal. How could she do that? That piercing would sit in her swamp folds and surely become infected, at which point she'd run back, screaming about it must be the shop's fault that this happened.