There are a good deal 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 people get involved in activities in exactly the exact same fashion as traditional tables and slots do. If the activity is free, then so is the law.
This is the reason the majority of online casinos are operated in the same way as casinos offline.

With online gambling, though, you do not need to leave your home, as long as you have access to your computer. All you need is a computer, an internet connection and a credit card. You may place a simple bet on the slot machines and walk away, or keep winning and playing until you get rid of all of your money.