you know, its really humorous you bring that up. I have the exact same annoyances. It came up in my CCNA class last semester, apparently the professor is a big fan of them and I tried disagreeing with him. Having been a network engineer for longer than my prof, I'm "that a$$h0le guy" that no one wants to be in class, but what the heck . . . I'm paying damned good money out of pocket and busting my hump going back to school at night, so I might as well get my money's worth of quality education, right? We were discussing trespassing banners as they apply to routers and such serving as a basis for hacking litigation.
Anyhow, he tried arguing the typical uninformed argument "if I quote real law, and put a non-disclosure statement in my email people are bound to it, right?" Well, having a passing interest in law, I've never heard of a case where a confidentiality clause in email was upheld in a court of law. I have heard of plenty of instances where some ignorant IT Manager thought it was a great idea, and forced their legal counsel to broach the subject in court. At this time I am not aware of a single instance where either the confidentiality statement in an email (or the BS: hack me and be prosecuted, because I say so statement on a router) has been enforced. Receive a private email because the person who sent it to you is an idiot and shouldn't have? Too bad, they're an idiot, you have whatever information they mistakenly sent you. Hacking into a corp network and read a banner saying you're a jerk and now broke the law? Well, that's true and you can and hopefully will be prosecuted to the fullest exent of the law, but that has nothing to do with the banner, and it is unneccesary.
Moral of the story? Just because some idiot CTO/IT/IS Manager/Director wants to put fancy legalease words on your email or router doesn't make them in of themselves legal. How you got to those words is what matters.