The last time I saw an NC-17 film in theaters (Shame, 2011), an usher had to walk through the theater every once and awhile with a flashlight of sorts, which I assumed was to ensure no 17-and-unders had snuck in to watch. Is this a MPAA law? Or is it a state law in Georgia? Or is it just a very diligent art house theater?
I'm not American and can't give legal advice, but I'd strongly suspect this was just a diligent movie theatre.
Cinemas are not legally bound by these ratings. They are simply recommendations made by the NPAA and seconded by the National Association of Theater Owners. To quote from the article linked:
In the MPAA's official Classification and Ratings Rules, all the sanctions aimed at violators deal with the companies that release films, not the theaters that show them.
“The rating system is voluntary, but we strongly encourage theaters to enforce the rating age restrictions as applied to any movie,” a NATO spokesperson told theWrap.
Therefore, there are no binding laws on the cinema in the first place, regardless of the film's certificate. I would suspect this is simply a diligent movie theater being careful about who they have allowed into the cinema. Alternatively, they could be checking for other things, including empty seats (for latecomers), ticket scalps, anyone recording in the film or other such things.