So if I understand what you're saying, it's that since there is no legal definition of art, I could call anything art if I wanted to prevent people taking photo

So if I understand what you're saying, it's that since there is no legal definition of art, I could call anything art if I wanted to prevent people taking photos of it. People would be afraid of the court fees, so they would just roll over. Is that a fair summary of your argument? I think you're right that this could happen, but this line of thought can lead you to doing nothing about anything due to the way it might be abused in the future. In this specific case, I don't understand what specific problem the law is trying to solve so I can't balance the positive and negative effects of the law against each other. However, it does seem to me a narrowly focused law with a reasonable definition of the subject.