Online gambling is legalized in the same way that any other type of gambling is made legal in the United States. A state government approves a bill to allow certain types of games to be played online. These games are regulated in the same way that games would be in a traditional casino. Gambling is legal in one way or another in 48 states of the country.
The two supporters who ban it in its entirety are Utah and Hawaii. Utah has a large Mormon population, which is a deeply religious community. It is natural that the personal beliefs of most residents affect local regulations. In Hawaii, there is a similar climate.
There, it is not so much the religious aspect at stake in the Aloha Islands, but the opinion of most residents that it will affect family relationships and the environment. Slot machines, and progressive slots in particular, offer the potential of six to seven figure paydays.