MAUS: Why did the Germans hang Nahum Cohn and his son?

In the novel Maus, Nahum Cohn and his son are arrested for selling goods without coupons; they are involved with the black market. After their arrest, they're hung in the middle of the black market center, where their bodies are left as an example to the rest of the people as to what their punishment will be if they're caught doing the same. Vladek is terrified by what had happened and doesn't leave his house for a period of time.