In fact, that usage goes back centuries, from English slang for a mean, low, "scurvy" rascal or scoundrel. As Stephanie Smith put it in Household Words:

From blemish...to strikebreaker, the history of the word scab…shows a displacement of meaning from the visceral or physical to the moral register...Just as a scab is a physical lesion, the strikebreaking scab disfigures the social body of labor.