If they want a photo of the set or anything production related, well, all I can say is that's what the souvenir programs are for. And, I do highly doubt that the reason people are taking photos is for their own amateur productions of the shows that they later want to do themselves.
For example, people at Waitress tend to take group photos after the show, using the set as a background. I stop them, sometimes mid photo and, for the most part, I've never had any issue at all. They've apologized for not knowing, and head out of the theatre. However, there was a group of girls, most likely in their twenties, taking photos of the set. When I told them to stop, they replied, "but everyone else is doing it." Granted, I wanted to reply back the old adage of, if everyone jumps off the Brooklyn Bridge... but of course didn't say that.
I guess the point that I'm trying to make is, just because you could take photos in that situation, doesn't mean you should. And, even if a ton of people are doing it, that still doesn't mean that it's the right thing to do. It's possible for a ton of people to do the wrong thing at the same time.