A sign-up bonus refers to an offer that casino gives to new customers once they sign up on their site. Casinos use these bonuses to attract new customers and provide them with some funds to start betting. Sign-up bonuses are not withdraw-able and should be used within a timelimit, depending on the casino.
The United States is the land of freedom and opportunities. Besides that, it's also where one can easily play casino games by joining many online mobile casinos that cater to US residents. If one doesn't have the chance to go to Las Vegas, online gambling allows them to enjoy their favorite games without leaving their home.