Just the fact that it brings in money is enough of a reason for the government to allow it. Not to mention the fact that most would argue about America banning gambling, when it has one of the most active and biggest casinos out there, not to mention that gamblers would argue that they are old enough to live their own lives and manage their own finance. And yes, I do agree with that, especially since I believe that America lets their kids know about planning and stuff at an early age, letting them live by themselves early on than later.
Let's also add that they won't ban a hobby they themselves are fond of doing. We all know some politicians are gambling addicts, and some gamblers or owners of casinos are connected to politicians making it harder to ban gambling in a certain country. Politics is dirty and all you need is money and connections, for you to influence about the outcome of a bill in the congress.