Oberon's Law is a magical decree passed by Oberon, which forbids the Third Race from directly interfering in the lives of mortals. It is unknown as to when it was passed, although it was already in effect by 1020 (and most likely was already in effect by the Children of Oberon's banishment from Avalon in 995, for that matter). The Law magically bars any member of the Third Race from being able to directly intervene in the lives of humans and gargoyles; even Oberon himself is subject to it. However, the key word is "directly"; the Third Race can find ways of getting around it, loopholes, if they are clever enough.
Thus, the Weird Sisters could still interfere in the lives of Demona and Macbeth by tricking them into consenting to it, for example, and likewise Anansi the Spider could turn Fara Maku into a werepanther because Fara Maku had sought him out and requested that Anansi do so. Oberon’s Law is the very reason why the Third Race act so subtly in their dealings with mortals; they have to, in order to avoid transgressing it.