Gambling is generally a regional thing and is subject to each state. Usually it’s governed by where you are currently.
Cruise ships on the other hand are governed by the country the ship is registered in and/or by whatever country they’re docked at.
So a law will state specifically that there is no private gambling allowed within the confines of a certain geographical area such as the state of Washington, for example. The law can also make provisions that gambling is allowed on Indian reservations or within certain city limits.
The cruise ship leaves the geographical area where gambling is not allowed and allows gambling to occur in international waters which is fully law-abiding.
Also, most states and governments will provide licensing for casinos. If you qualify you may proceed with gambling in that jurisdiction.
People will probably think I’m the most moderate right winger ever. But I’m really not. I just have hard opinions on things.