I think there's nothing much you can do now, from a legal point of view.
It was specifically mentioned No Camera for obvious reasons -- possible infringement of copyright laws.
But in the first place, I think:
1. You should not have exposed your camera in front of any of the staff. Never mind that you're still outside the cinema. It doesn't matter if you're a 100% honest guy who will not take any shots inside. The point is, you had the camera and they certainly couldn't give you the benefit of the doubt.
2. When you handed over your camera to the guy, there was no agreement that he couldn't touch your camera. It's like you leave your bag in the locker and the notice says something like "The management will not be responsible for thefts, etc."