There are a lot of legal reasons why online gambling is legal.
First of all, online casinos aren't places where people gamble for real money, rather they are virtual websites where folks take part in activities in exactly the exact same fashion as traditional slots and tables do. If the activity is free, then so is the law.
This is the reason the majority of online casinos are operated in precisely the same manner as casinos offline.

With online gambling, though, you don't need to leave your house, so long as you have access to a computer. All you need is a computer, an internet connection and a credit card. You can put a simple wager on the slot machines and then walk away, or keep playing and winning until you lose all of your money.