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If an employee spends work time on personal activities or entertainment, the employer will obviously not like it. And it makes no difference whether you are gambling, scrolling through your favorite social network or something else. In your free time you can do whatever you want, but at work you just have to spend all your time on work.
Whether to follow it or not everyone decides for himself, but we all know how nice it is to do our own things.
It makes no difference because both are violating the rules of the company. however, in terms of effect, gambling could have a big effect especially if an employee is getting addicted with gambling as it could result to stealing some company funds especially if he has some access on cash like being the signatory of the company working as a manager or something. I'm sure you heard stories about an employee slowly but consistently stealing money from the company to fund his gambling addiction, but with playing solitaire or any game as long as you are not gambling, that is not gonna happen.