Because, it is told earlier that to justify is to show reason. That is to say, to justify a statement as true is to show that why it is true. And it is only a statement or an assertion what involves reason. While it is not reason but aim or purpose or intention an agent may have in his mind to do an act. An action can be evaluated and it is an evaluation what we justify.
Punishment is a kind of action.
We can evaluate it as just or unjust. And such an evaluation can be justified. So, it is not punishment, rather statements concerning punishment what we can justify. I owe to Pro. Ibid , P.82.