The point isn't whether it's genuinely plausible or not. If there is some law against something, there is some group of people who believe very strongly that this law should be followed and enforced. If the law is not actually followed or enforced, then the system as a whole does not agree with that small group. The point of this "plausible deniability" is to allow whichever regulator is concerned to give a pass to rule breaking without explictly doing so and hence angering this small group.