If we stated that (say) criminal laws could not be enforced unless the defendant had access to the full body of possible codes and precedent before the crime occurred, we would literally never be able to convict a single case, ever. Because any defense attorney could just find some arcane memo and prove that the defendant could not reasonably have been expected to have access to it before the crime occurred, and that would be sufficient for excusing them of culpability. And that's not even raising the question of whether or not they could reasonably be expected to interpret and understand the text, which would be the next hurdle. (The same applies to non-criminal cases too.)
(Persuasive precedent is not binding, so it's not "law", but it's undeniably influential enough that it's necessary to understanding the law.)