Why have the narrator's and Lou's parents been named "enemies of the people"?

The narrator's parents are doctors, and thus, named enemy of the state due to their education, previous practice mand class in society. Luo’s father is a well-known dentist who was labeled a class enemy after revealing that he worked on Mao’s teeth. The narrator recalls a time when Luo punched him in the face after Luo's father was publicly humiliated.