We can only obey the rules if we want to still work in that company. We can't risk our job by doing something that is prohibited by the company. Besides that written rules, we should care about the unwritten rules that may be not allowing us to do something. It is better we work carefully so nothing bad will happen to us and we don't get any problems. But if we do something by coincidence, maybe the company can forgive us but they will give a warning not to do the same thing in the future. So if we can be careful while we work, we can be a good employee and the company may reward us with something good.
Have you read the article? That fired person wasnt gambling at work or during work, but the reason to fire him was gambling. I have never heard that any company has "any form of gambling is forbidden everywhere" kind of rule. For example, if gambling at home is not acceptable by company, then this is a total nonsense.
I think it would be appropriate to warn your employees when hiring them that they should not play in the workplace because in the worst case scenario it could lead to dismissal. If the employer did not warn about this in advance, then I think this is not the best option for dismissal, because an employee can be fired under any other pretext and even if there is no reason, a higher-up can deliberately make it up because of personal hostility, for example. In general, I think that the first point in this case should have been a warning, not dismissal, this seems too harsh. I wonder if they have trade unions that could prove that this did not affect the employee's work and the entire bank as a whole.
You got a good point, if that's not a part of your contract then dismissal should not be the first option, there's should be a warning first before dismissing an employee as you mentioned it can easily be abused by any higher authority in that company since they can quickly decide to punished you with dismissal if they wish using that kind of reasoning, though it's tough decision to make since employers or any head of the company have that authorization if that's part of the contract then there's nothing to talk they can simply implement the dismissal.