People precommit to punishing criminals because having a precommitment discourages crime--if the criminal can figure out that you'll change your mind once the crime is over with, and not punish them, the prospect of punishment won't work as a deterrent. But if you have to follow through on the precommitment, that may mean you just "cruelly" punished a criminal "even though at this point causing the criminal harm doesn't benefit anyone".

Most people don't consciously think of it this way, but people don't usually know why their own memes spread.