I guess it's the double standard many criminals have. On the one hand, they wanna make money by any means necessary - on the other hand, they've a reputation to keep up. That's probably why Narcisse sells the smack in AC, not in Harlem. When Purnsley comes to his office the Dr. is very upset that Cotton Came To Harlem.
Harlem seems the place for his "Lybians" to strive, hence the beating of the foul mouthed guy.