life in prison is so hard... there you will be able to find various kinds of things that should not be allowed, such as drugs or gambling, those are the things that are most often found in prison. why gambling can be so free in prison is because the prison boss bribes the prison guards and allows them to play freely without interference. usually the prison boss has full control over gambling in the prison, and he is also responsible for drugs, cigarettes, women and many other things. so even if you are in prison, gambling will still exist, in fact it is much more massive in prisons where security is not so strict.
But, do you know that it's not usually the same thing in every prison, like some countries, they will not allow drugs in the prison because if noticed, the boss in charge of the prison could lose his job and the prisoners involved would face more difficult sentence, even some wouldn't allow gambling. So, it totally different everywhere. I know that in movies, we see different things that might not completely happen in real life.