Gambling is everywhere, so I didn't get shocked if there's gambling in prison even though it's prohibited.
Even with recreational activities like sports, weightlifting, or board games, they can still bet without their warden knowing.
AFAIK, inside the jail, they have many activity programs that might not include gambling, plus work assignments that keep them busy and religious activities that help them become more serious.

I'm glad that I've never been to jail in my whole life, maybe just because I followed the 10 commandments, which makes me afraid to commit a crime.