The problem is always the guards, once a powerful figure enters the prison its easier to confuse the guards to bend the kneel for money, sooner it will turn into some business and even the guards can be used to send out information.

Forget how they will get paid, there is always a way for criminals to settle those guards, they also have families at home that needs to be feed, so gambling is just one of the easiest thing that can happen in the prison.

The issue about this is how unsafe this can be, the same betting cards they use in prison can be used to send out commands or strikes right from the prison, or other vital information, it should be wrong but it is what it is.