For sure this is happening on 3rd world country prison. Here in PH even on prison they have a drug den and private house with complete amenities for drug lord that still can operate their drug business even inside the prison.
why would you think that first world countries do not have the same thing? they might even have more. there are plenty american shows about prison and they are not as organized and clean as others might think. there is also some corruption inside.
In fact, all businesses are in prisons and become untouched places by outsiders but can have a lot of access if they have a lot of money.
It is no wonder that gambling in prisons is a part of their daily activities and there are many more illicit activities.
In some developing countries even the warden who was supposed to be a prison guard, but instead became a drug dealer, it was quite shocking.