For the player, if online gambling is illegal, it's illegal under state, not federal law. In my home state, it's illegal or at least our governor thinks so.
This
web site has a state by state matrix that should tell you what you need to know. I cannot vouch for it's accuracy, but I've had reason to follow this topic lately and this web site seems a bit less off the wall than most of those that discuss online gambling.
With some of the other web sites I think the motto might well be: "They can have my electronic straight flush when they pry it out of my cold dead hand!" You would think people were trying to take away their AK-47 or something!