I would imagine it often comes from censorship although I can't give a 100% answer. I buy poo a bit on sites like Pantydeal and All Things Worn over the last 5-6 years and each year they block a new search term for poop. Sellers adapt, come up with a new way for their product to be found, buyers see the new way and adapt. Fetish site admins censoring a completely legal fetish because they themselves don't like it is wild to begin with, but probably common for scat related topics