Irrelevant. There doesn't need to be any.

"What's the financial motivation for the state" is way too blinkered and makes at least 2 different false assumptions. There are other motivations besides financial, and it doesn't have to be the state's own motivation for the state to end up doing something.

Countless government policies and programs exist which give no legitimate benefit to the state or the people, no legitimate motivation, financial or otherwise, yet they exist anyway because they do benefit and motivate someone, financially or otherwise, who has some mechanism of influence to cause it to happen.

This is almost like asking "Why would anyone do something bad?" Gosh golly why indeed? 517,000 reasons and 124 new ones every day.

Relevant in that the directly upthread poster argued there was “zero financial motivation for the state for prevention or rehab or any other activities to reduce imprisonment rates”, which I claim is incorrect.