Why you shouldn't use soap to clean a pizza stone? or some pans and pots?
Some cookware are 'cured'. Mostly cast iron pots and pans. I never heard of a pizza stone being cured, so I'm guessing this is to avoid eating soap. Other than that, I wouldn't know.

Cast iron gets cured for two reasons: 1. Create a non-stick layer 2. Avoid oxidation. Neither of these reasons apply to stones (or do they?)