Gambling became legal in many countries because the government realized that they are wasting a possible huge income earner for their country. Rather than gangs and syndicate taking advantage of gambling profit, they then decided to adopt gambling as a legal industry.
Well, i think so you need to explain your point of view in more words as i cant see a good reason for the gambling to be legal for the government as gambling is just a recycle of money no service or product is being produced so thats not gonna help in the economic cycle. Gambling is legal in some of the countries as they want to keep gambling in control so thats a good reason i can see behind it.