One of the cornerstones of any legal system - for it to deliver justice - is that what is and is not permitted or prohibited is known clearly in advance by all citizens. Thus allowing a decent law abiding person to conduct themselves in such a way so as to not break the law.
If you remove that precept, you have kicked away the chocks that hold the ship of society safely, and begin a slow descent down the slipway until the whole edifice crashes into a sea of tyranny.
There is even a legal precept "dog-laws", after Jeremy Bentham (not someone you'd instinctively refer to in such cases) ...
"It is the judges (as we have seen) that make the common law. Do you know how they make it? Just as a man makes laws for his dog. When your dog does anything you want to break him of, you wait till he does it, and then beat him for it. This is the way you make laws for your dog: and this is the way the judges make law for you and me. They won't tell a man beforehand what it is he should not do - they won't so much as allow of his being told: they lie by till he has done something which they say he should not have done, and then they hang him for it.
Be interesting to see if it's cited tomorrow
. Call it my pro-bono contribution.