Some Korean shop owners were certainly racist towards black customers, but I’ve never read any accounts which suggest the protestors were selectively targeting racist shop owners. On the contrary, most accounts I’ve read seem to suggest people were taking out their anger on Koreans in general.

In principle, I think that collective punishment is wrong. I don’t see much conceptual difference between targeting all Korean stores because some owners were racist and targeting Muslims because of a terror attack carried out by one group of islamist radicals.