The stone has pores in it. They say that the soap can "soak" into those pores. When you heat them up during cooking it can give your food a soapy taste.
I have never used soap, but for those that have, it there any bit of truth to this?
I would guess that you can use a soapy rag and immediately wash the soap off, but don't soak the stone in the sing with the rest of the dirty dishes. Just a guess.