The Punishment Defines the Crime
What happens if you stick your finger in a candle flame? You get burned. The pain could last for days, and your finger might never be the same again.
Is that what you deserved? Wouldn’t a brief moment of pain, followed by no lasting pain (once you removed your finger from the flame), and no permanent injury, be enough? That would probably get just about anybody to jerk their finger out of the flame, and not stick it back in. Isn’t the actual burn overkill? Much more severe than necessary?
No. It isn’t. The degree of caution that people will exercise in the vicinity of a candle flame is much greater because of the degree of harm that it causes. If the candle flame only caused a moment of pain, with no lasting injury, then you wouldn’t be so careful around it.
So, from this example we can see that the punishment doesn’t just fit the crime, it actually defines the crime.
This applies even when the penalty is death and the criminal is a minor. For example, if a whole city of people, including children and infants, lives in Pompeii, near the lip of an active volcano, and it erupts, killing them all, then the severity of the punishment is exactly what it needs to be to let everyone else in the region know just how important it is not to build a city there.
And it shows the survivors just how important it is to figure out what warning signs indicate that a volcano will erupt soon, and how important it is to have a plan to deal with an impending eruption.

Update 2009.10.27 — This is also why even today the law still employs the centuries-old principle, “Ignorance of the law is no excuse.” In reality, of course, ignorance of the law is a perfect excuse. But the law isn’t trying to give anyone what treatment they deserve, only control the population-in-general so that general prosperity and national strength are maintained. If you didn’t know something was illegal, society finds it most expedient to penalize you anyway, making an example out of you, for others to observe. How this affects your individual feelings and future actions is irrelevant compared to the tens, hundreds, thousands, or even millions of persons who are strongly deterred by seeing what happened to you.
Update 2009.11.24 — This same principle applies in the other direction: to unenforced or inadequately enforced laws. If there’s no punishment for doing X, then X isn’t a crime. Period.
