In fact, in many countries around the world, prisoners gamble despite prohibitions by law and local authorities. Of course, there are conditions of detention that physically prevent gambling, for example, for prisoners in solitary confinement under strict conditions. But in general, gambling is one of the ways prisoners entertain themselves in this harsh environment. Incidentally, many card games originated in prisons. And to be honest, I don't think gambling can be eradicated in prisons.
In prison, they are punished based on the crime of each person, so the condition of all prisoners is not the same. Some prisoners are even kept away from entertainment, they are so dangerous.
And besides, you will find many gamblers in prison who are addicted to gambling and have been caught committing crimes while funding gambling. Considering all aspects, I think it is better to prevent them from gambling.
It is not true that Google gambling is the only source of entertainment, so if they want to be entertained, other sources of physical entertainment can be made available to them.