So, this comic is milking the question for drama because of course it is, but there's an argument to be made that the rules against "humans becoming youkai" that the fortune teller ran afoul of only really apply to the villagers. The fortune teller was killed because it would be dangerous if people started getting the idea that every Tom, Dick, and Harry could become a youkai, but if some weirdo who lives by herself in the woods like Marisa becomes a youkai, then that's just to be expected, isn't it?