I think it's not illegal and legal because there are no rules about it.
There is/are rules about it. If you are living in a country that is religious then gambling is no doubt illegal on it but if you're living in such a nice country that is allowing people to gamble and they are taxing people's profit and as well as the establishment then there's no need to worry on it, they are legal on those places.
I think it's not only about religion. Evolved countries allow gambling (and they are religious/conservative), while poor countries don't. Evolved people know very well how to deal with gambling on their countries, they can live with it without becoming addicted and without ruining their lives and their economy. I really don't understand why poor countries are against gambling as it could generate an interesting income for the public coffers.
Wrong, most of the countries that aren't wealthy enough does have a lot of gambling place that exists. And to those countries that aren't allowing it are the ones who are mostly rich and doesn't want to let their citizens involve with such activity because they know what will be the return to their people.
I'm not sure... USA is a rich country and allows gambling, some principalities (that are wealthy places) the same. Now look third world countries, muslim countries, latin america countries, they don't like gambling and don't allow their citizens to play. Gambling is benefical if regulated in the countries as it will return money in services to the population (the basic services).
I think you have made a blunder by making a statement that USA allows gambling because she is a rich country. I completely disagree with you in this regard because gambling is a bad habit as its image is bad all over the world and definitely, the country who would be promoting it would also be considered bad as it is very stressful and disastrous for the users and no country would like their reputation spoiled due to this fact.