Some rules just seem natural. Don't scream fire in a crowded theater. Call 911 if you witness a crime. Tip your waiters. Pay a psychic top dollar for the more accurate predictions. Okay maybe not that last one, but you catch our drift. Now, we aren't saying these are bad rules (except that last one), but we are saying that The Island of Dr. Moreau takes issue with society's rules, and specifically with the idea that they're natural. They are, in fact, anything but natural. They only feel that way because they are so deeply integrated into our daily lives that to think differently seems unnatural. We can see this is the Law of the Beast Folks—completely natural for them, utterly oddball for us. So the question becomes, what rules are natural and what rules aren't? And how do we tell the difference?
Prendick mocks the rules of the Beast Folk because the ceremony is so foreign to his British sensibilities. Had he listened to the Laws, he would have seen they are similar to his own rules.
The Beast Folk actually represent a civilization. They took the laws from the Kanakas missionary and made them their own. It gets weirder. They actually want Moreau to enforce the Law for them.