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 gargoyles and humans; 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, for example, the Weird Sisters could interfere in the lives of Demona and Macbeth by tricking them into consenting to it. Later, when both Demona and Macbeth were suicidal enough that they had effectively abandoned control of their own destinies, the Weird Sisters took advantage of another loophole.  As of 1997, Oberon remains unaware of the Sisters' interfering with Demona and Macbeth's lives.  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 also applies to other mortal animals.  Despite this, there are existing loopholes. Anansi's spider servants, for example, might be content with their participation that the spiders' interactions with the Trickster Spider adequately get around the Law. Further, Oberon himself may have been too arrogant to consider that mortal animals other than gargoyles and humans would be included. 
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.