The line isn't as sharp as you might think.

Certain private spaces are considered "public" in some regards, such as right of access and some free speech rights (e.g. handing out pamphlets in malls).

Also some "public" spaces are privately owned (e.g. the park where Occupy Wall Street got its start was supposedly open to the public as a condition of getting some building permits, yet the landowner spent money evicting the occupiers).

The casino isn't free to bar people of certain races, and it may have other restrictions as well. Though spying is presumably unrestricted :-(.

The line is extremely sharp in this case.

None of the things you refer to has any meaning as it pertains to the private spaces within privately owned casinos. A casino lobby in Las Vegas is not considered a semi-public space like a mall. That was settled a long time ago (it's why the porn pamphlet people can't go inside the MGM and pass those pamphlets out in the lobby).

All the privately owned casinos in Las Vegas can legally do this. A challenge to it in court wouldn't get anywhere.

They can't legally put hidden cameras in their toilet stalls or hotel rooms, why would they be able to legally put systems that can obtain a nude image of someone without their consent on the casino floor?

If the casino owner were going around the taking furtive pictures up girls' skirts, would you still think it was clearly on the up-and-up? The fact that it is private property doesn't mean you have unlimited rights to do what you will to other people there, particularly without their consent.