J.S. Mill's Puzzling Position on Prostitution and his Harm Principle

Abstract J.S. Mill argues against licensing or forced medical examinations of prostitutes even if these would reduce harm, for two reasons: the state should not legitimize immoral conduct; and coercing prostitutes would violate Mill's harm principle as they do not risk causing non-consensual harm to others, their clients do. There is nothing puzzling about Mill opposing coercive restrictions on self-regarding immoral conduct while also opposing state support of that conduct. But why does Mill oppose restrictions on prostitutes’ liberty if those restrictions could prevent harm to third parties? Mill's position is not puzzling once we recognize that his harm principle is not a harm-prevention principle that warrants restrictions on liberty to prevent harm no matter who caused it (as David Lyons famously argued) but instead warrants restrictions on liberty only of individuals who are the morally relevant cause of that harm. Mill's discussion of prostitution shows he prioritizes both individuality and moral progress over harm reduction.