the prison houses a lot of people that are hardened and that will not always agree to keep to their own side of the bargain. even if officially gambling is not allowed in prison, if it is allows to thrive, it will lead to a lot of fighting and chaos because of the kind of gamblers you will mostly find in prison environment. some local prison that have officers that are really corrupts can allow prisoners access to some form of gambling but the fun will be almost close to nothing because of how regimented the prison environment is.