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.
If I am not mistaken, I think they are hosted by high high-ranking officials. Since there are programs for prisoners in my country to earn money, it is probable that the authority who is in charge of these prisoners wanted to milk them of their income so these people allowed gambling and other sorts of illegal activity inside the prison. I heard a rumor in some cases that convicted criminals who are already in jail gain temporary freedom in order to execute targeted people.
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.
This is a huge source of money for the corrupt officials, so they will definitely allow these kinds of transactions.