The answer to the first part of your question will vary from state to state within the United States. Different states have different laws about gambling. I live in Washington. In this state, casino-style gambling, with things like slot machines or roulette tables, is only legal on Indian reservations. There is a state lottery which is, of course, legal. Card games like poker and blackjack can be held outside of Indian reservations in some circumstances (see link below for more details).
The answer to the second part of your question is a matter of personal opinion. My own opinion is that gambling should be allowed on Indian reservations if it is not allowed in the rest of the state. My reason for saying this is that I think that gambling should be legal in general. Gambling is certainly not a good thing. I do not gamble and I do not think that it is good for other people to do so. However, the government should not be in the business of banning things just because they are bad for the person who does them. There are many things that I do not approve of, like smoking and drinking, that are legal. If people want to do things that are generally bad for them, it is their right to do so. I do not see where it makes sense for states to ban gambling simply because many people disapprove of it on moral grounds. Because I think gambling should be legal in general, I think that gambling should be legal on Indian reservations when it is banned in the rest of a state.