Most venues at the 350-seat size already do have a no-recording policy, usually clearly posted in the lobby. The point already made to be clear about terms of ticket sales is a good one, i.e. try to minimize the expectation that recording may freely be done. Sometimes there is an announcement at the beginning to remind people not to record (or take pictures if applicable. However that doesn't stop some people recording with their phones, or simply using their phones to talk or text surf during a performance, which is more distraction to audience than performers on a lighted stage.

If one is going to do something so blatantly illegal it amazes me they don't set their screen to the dimmest possible setting to avoid attracting attention. Peer pressure only works for behaviour that is generally agreed by society to be immoral. Apparently theft of music is not in that category.