Maybe in prison they are not betting with money rather some task and chores that assigned to them since they don’t care about money when you’re in jail.
This is all with the jail warden's knowledge. If the jail warden is not corrupt, how come these things will be present? Jail warden often let these things slide because they benefit from it. And as far as I know, inmates in the Philippines can earn some money due to the livelihood projects that were established there. So inmates can gamble with the money they earn inside the prison.
Btw, prisoners do care about money since it is their ticket to talk to their relatives, since some wardens make it a business to let inmates pay some amount to convey a message or even call their relatives. As long as the price is right, inmates are allowed certain things that are prohibited in prison.