"He's doing therapy, too. That's mandatory. But he's also working in what you'd probably call a penal colony, or he will be, when he's stable enough."

Which is also an option for Khan, but not one Kirk remotely wants him to choose.

"He has it rougher than you do. He earned a tougher penalty than you did, and Starfleet knows that. So I told them to let you choose."

A lot can change in a month for Kirk, especially when he's actively taking the advice of Khan's counselor.