Gambling is indeed a business type with high externalities per economics, but the reason gambling dens and pubs get away with this is because of money. They bribe authorities, sometimes the government allows them to operate within their borders so long as they pay tithings, these taxes are supposed to counteract the externalities the gambling den will incur during its operation, yet of course only a small percentage of the tax ever goes to where it's supposed to courtesy of good ol' corruption. These for me are the biggest reasons why even though allowing a gambling den to operate is detrimental to the society, it is still allowed to operate anyway.
There are also casinos where you do not have to pay taxes. I don't know about whether the casino has to pay taxes, but then it wouldn't be too good for the economy. Online earnings often disappear on offshore accounts. Then the government gets it all wrong.
Most casinos are obliged to pay their portions to the government in the form of taxes, those who do not either operate behind the government's radar or had simply paid enough mouths to shut up during meetings. Either way we must agree that these types of business-models shouldn't be allowed because not only is it damaging enough to have a gambling house operate at its leisure within your country, but the fact that they don't pay as much locals would.