Some bitcoin casinos are legal. If a bitcoin casino is operated in the same manner as a traditional casino, it will fall under the same laws as if it is a traditional casino. There are no federal laws against online gambling. However, there are some state laws prohibiting the practice.
Where ever the gambling is banned there bitcoin gambling is also banned. but no one can find us easily so people play bitcoin gambling and there is no legal and illegal matter in online gambling.
You have different kind of regulations in different countries. I think in some islamic countries gambling is forbitten in general and this totally independent of whether for money or not.
Then you have countries where it is forbitten to gamble for money, but if bitcoin is not considered as money and in most countries it is not, then gambling for bitcoin is legal.
However, there are few gambling sites like Jetwin where you can play with anonymous accounts and bitcoin is basically anonymous as well.