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):
Imagine a country with two rules: first, every person must spend eight hours a day giving themselves strong electric shocks. Second, if anyone fails to follow a rule (including this one), or speaks out against it, or fails to enforce it, all citizens must unite to kill that person. Suppose these rules were well-enough established by tradition that everyone expected them to be enforced.
So you shock yourself for eight hours a day, because you know if you don’t everyone else will kill you, because if they don’t, everyone else will kill them, and so on. Every single citizen hates the system, but for lack of a good coordination mechanism it endures. From a god’s-eye-view, we can optimize the system to “everyone agrees to stop doing this at once”, but no one within the system is able to effect the transition without great risk to themselves.
Eliezer Yudkowsky offers 'Tolerate Tolerance' as a dictum against punishing non-punishers:
That's why it's so important for us to tolerate others' tolerance if we want to get anything done together.
(...)
Cooperation is unstable, in both game theory and evolutionary biology, without some kind of punishment for defection. So it's one thing to subtract points off someone's reputation for mistakes they make themselves, directly. But if you also look askance at someone for refusing to castigate a person or idea, then that is punishment of non-punishers, a far more dangerous idiom that can lock an equilibrium in place even if it's harmful to everyone involved.
Anti-social punishment, punishing those who try to do good, is a related idea (See also, Looking Too Good by Robin Hanson).