So, what happened to ignorance of the law is no excuse?
Mostly, it's been misunderstood. Ignorance
of the law has never been a defense for breaking it. This basically an ancient principle, based on the premise that legal prohibitions exist to preclude
bad acts. Laws aren't created just because it's fun to legislate or to prosecute. They exist because
bad acts are prosecution worthy. So, if you murder someone in cold blood, it doesn't matter that nobody told you murder is illegal. What matters is that you intentionally murdered the person.
But many uneducated idiots, especially internet arm chair lawyer types, have completely misconstrued this legal principle that reached back to antiquity with the legal concept of
intent. For many particular crimes, there must be some kinds of
mens rea, which is Latin for guilty mind. In other words, a particular action must be undertaken with some kind of intention to engage in a particular
bad act.
For example, let's say that there's a shiny red button in front of you on your desk. Somebody tells you "Don't push the button", then walks away. Ten minutes later they come back again and repeat this exactly, and every ten minutes thereafter, for about an hour. You have no idea why they're saying this, and after a while you're feeling cheeky, so you push the button. Unknown to you, the button controls a ventillation system in an airtight room a mile away. The air gets sucked out of the room, the man inside dies. Is this murder? No. Your act of pushing the button lacked
mens rea. You had no good reason to expect that the
actus reus of pushing the button would produce the effect of killing the man inside the room.
On the other hand, let's say the person starts off by actually explaining what the button does, throws up a computer screen showing a live feed of the airtight room, and connects you on a video call with the man inside who also tells you what the button does. Everyone involved also tells you that the man is actually somewhere in a remote part of the world that has no government and therefore there's no laws against murder that would apply. And after all of that, you decide to push the button and then sit there and watch the man on the screen die of suffocation. Is this criminal murder? Yes. Because your act of pushing the button was done with a
mens rea to kill the man.
So the moral of the story is that
ignorance of the law is no excuse for criminal conduct. But
lacking mens rea can make an action noncriminal in the first place.
I know it's all very complicated, and is nowhere near as simple as counting change when working at a fast food joint. But that's why it takes a really long time to go to school long enough to become a lawyer.