Punishing Non-Punishers describes the act of punishing people who don't punish someone else that you punish or think should be punished. It is used to make the punishment of the target more severe and to increase conformity among everyone else.
There's more than one level of punishing non-punishers - you can punish those who don't punish non-punishers, you can punish those who don't punish those who don't punish non-punishers, and so on. Taken to the limit, it becomes "If you not with me, then You're my enemy".
If a crime is sufficiently bad, punishing non-punishers can be appropriate, but otherwise it's an incredibly dangerous dynamic. Nick Bostrom describes an hypothetical scenario where punishing non-punishers is used to maintain a maximally bad equilibrium (described below by Scott Alexander in Meditations On Moloch):...