There can be two different kind of restrictions from what I know:
1. The game developer is not allowed to operate in a certain country at all. Each casino that hosts games by that developer has to restrict players from those countries from playing those games.
2. The game developer is allowed to operate in the country, but casino A has not acquired the needed licenses for that jurisdiction, and can therefore not host the games. Casino B has the needed licenses, which makes them eligible to run the games on their site.