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Google AdSense Program Policies
Site may not include:
Hate, violence, racial intolerance, or advocate against any individual, group, or organization
Illicit drugs and drug paraphernalia
Pornography, adult, or mature content
Gambling or casino-related content
Pop-ups that interfere with site navigation or are for downloads
This also begs the question of why Google is allowing these advertisers to show up in Adsense ads. If they won't allow gambling content, they shouldn't be serving gambling ads.
As for Adsense, I specifically asked Adsense support about one of my sites that was "marginal" in the sense that while it didn't have any gambling/casino stuff going on, or even any links to gambling casino stuff, it did have reviews of book content that was about gambling/casino games/gambling addiction/probability math, etc... and while they agreed that it didn't really have gambling and casino content on the site itself (it was about casino books, not casinos), it was related enough that they didn't want me to include adsense on it.
I suspect that if they ever notice the site you are talking about, the webmaster will either get a warning or a cancel notice. Since Casino content is one of those traditional areas for high-spam/scummy/illegal sites to hang out in, I can see why they trust their SERPs to be ok to advertise on, but not the general run of gambling/casino sites.
The ads that show up are not casino ads but card games type ads. Books on card games, etc.
They emailed me back with a couple of questions before they approved them. Makes me think they are pretty strict on those categories.