The devices on display are set up explicitly so that the public will have access to (and in fact are encouraged to explore) their features, which includes the webcam on them.
This to me sounds like implicit permission to use the cameras, as well as implicit permission to install software. Any legal line this man may have crossed is beyond the act of simply using the camera, or installing software. He had implicit permission to do those two things.
I doubt that. You can use the webcam on them to take photos (using Photo booth, etc.), but to suggest that a customer is implicitly allowed to install software that surreptitiously photographs other customers and then displays them in public is ridiculous. If what you suggest were the case, then that would mean Apple is implicitly allowing people to install keyloggers or similar malware. Just because Apple didn't post a sign saying, "don't install surveillance software, malware, worms, viruses, trojan horses, etc." doesn't mean that people should assume they are allowed to do that; common sense says otherwise.
Apple is providing its store guests with computers to use for reasonable purposes. I can't think of any way that what he was doing could be considered to be reasonable. In fact, it was closer to harassment (for Apple's customers) and spying (and repeatedly, since he admits he had to repeatedly return to reinstall the software after it was erased). Thus, his newfound trouble with law enforcement.