I personally think that Rock should be weak to Poison instead of resisting it. When you spill poisonous liquid onto a rock, the rock will slowly dissolve, similar to the reason Water is good against Rock.
A logical answer will do for this question.
Both rock and ground are capable for decomposing anything! Moreover, rocks won't get affected by toxins. And water is strong against rocks because a somewhat strong water current is enough for blowing away rocks within a movement!
Source: My experience and this