Gambling is prohibited in financial institutions due to the possibility of harm to the bank and other bank employees by gambling addicts or gambling employees. In South Korea, gambling is not only prohibited in one institution, but gambling is strictly prohibited for the entire country. Even though you are a South Korean bank employee, by engaging in gambling, you are not only violating the laws of the institution, but you are also violating the laws of the state. Gambling is prohibited in many countries of the world and there are strict penalties for gambling. In this case, I do not see anything special about a verdict against a bank employee in South Korea.
It does not matter what kind of currency he uses to gamble, he is a 'gambler', that is enough to rule against him and I think it is right. If he was not involved in gambling, the court would not have ruled against him or it would have been unfair to him if it had been ruled against him.
You sound like being a gambler or gambling in South Korea is a sentence to be jobless and cast away. And you are not fully true about gambling restriction is South Korea. Expats and tourists can gamble legally in any casino.
I still think that is unfair to fire a person because he has gambled at work. Employer must evaluate his performance at work, but not his hobby and how he spends his time. If he fails to do his work, after a warning he can be fired. If he does his work perfectly, and for example gambled on his mobile during lunch, then bank has no right to fire him.