No wonder if prisoners can playing gambling inside the prison because they can bribing the guards easily to allow them doing many things. Maybe gambling for them is just for entertainment but sometimes, they can share the info from the outside or running their business. It happens in many prisons where many prisoners can still lives comfort like in outside.

Smuggling something from outside to inside of prisons is usual things to do so that will not be a problem for them. Bribing the guards is also help them to get what they want from the outside.