Feinberg’s question is what distinguishes punishment from mere penalties. His answer is that punishment has an expressive function that penalties lack.
Feinberg then uses the expressive theory to smoke out sneaky attempts to punish people while avoiding the normal legal constraints on doing so, to explain why strict liability is tolerated when fines are the punishments but not when the punishment involves imprisonment, and to solve a problem with the justification of punishment that the retributive theory fails to solve.
One problem with retributive theories of punishment is that it seems to be impossible to meet the necessary condition: only the guilty can be punished. For example, innocent family members are often hurt by punishing the guilty.
The expressive theory can’t remove the harm that the innocent suffer. But it can help us to make progress on the apparent incoherence of punishment. Because we are not expressing our hostility towards the innocent family members, we are not punishing them, even though we do harm them, when we punish the guilty.
So it’s not possible for punishment only to harm the guilty. But it is at least possible for punishment to be restricted to the guilty.