I was also curious about this statement. According to wikipedia, one third of the Plötzensee Prison inmates are there due to fare evasion. Or rather, they were when the articles that wikipedia cites were written. Of course, being a juvenile prison in Berlin, it might have the highest rate of fare evader inmates of any prison in the country (which would explain why the articles write about that prison specifically).

https://en.wikipedia.org/wiki/Plötzensee_Prison

https://taz.de/135-Schwarzfahrer-hinter-Gittern/!5134051/

https://www.tagesspiegel.de/berlin/jeder-dritte-in-plotzense...