Yes, bookmaking is generally illegal in the US.
In his May 2018’s order, US Supreme court allows States other than Nevada to provide bookmaking, betting at casinos and racetracks. Due to the absence of Gambling legislation, it is illegal to operate a betting game in USA except for Nevada, New Jersey, Oregon, and Delaware.
Other than Nevada, Delaware is the only State where you will find both online betting and sports betting. And, no online bets for Nevada Bookmakers.
Remember, the NJ accepts legal wagers on sporting events. Also, the State of Oregon allows gambling on licensed horse, dog racing, social gaming, charitable events, and Indian casinos.