That's possibly true. However, having had the experience of someone show up to a disciplinary meeting to argue that he couldn't be sacked for spending hours a week looking at porn on his work computer, because there wasn't a specific line item in the workplace manuals or codes of conduct saying so, you'd be surprised at what people will argue to avoid getting fired when they're doing something anyone without some serious mental or moral impairment would recognise as not OK.

Or, to put it another way, every bizarre rule or sign usually has a story behind it.