There isn't and this is what many people do. But that isn't the argument is it? Can you tell me why observing what people do on my website is unethical and why it should be made illegal?
Also can you explain why information that is placed on my server belongs to someone else if I didn't enter into a contract with them specifying that was the case?
If someone has access to your laptop and downloads their photos onto it, who do those photos belong to? Put another way, if someone shows you a photo, who does the memory of that photo belong to?