This follows basically the same distinction as the examples using “classe” in the lesson. 

Prison is treated as an abstract noun when referred to generally. We have a similar distinction in English  - we wouldn’t use an article in the abstract situation.

 Le voleur est en prison/the thief is in prison (imprisoned or incarcerated)- is the example in the lesson.

If referring to a specific physical place eg Il est dans la prison où l'émeute a eu lieu/ He is in the prison where the riot occurred. 

We would use an article in this context in English.