This is untrue. An email written by you is your copyrighted work, from the moment you type it. Just because it's in my inbox doesn't mean you've relinquished your copyright. Technically, if I republish your email to me, you can sue me. [1]
The other day someone was polite enough to ask for permission to republish one of my comments here on HN on their blog. That surprised me for a second, but it shouldn't have. They were doing the correct, legal thing -- my comment was (and is) copyrighted by me; unless they were reproducing it for a fair use -- as I just did, when I quoted your post for the purpose of commenting on it -- they needed my permission to reuse it.
Now imagine how useful Facebook would be if every user had to seek a copyright release from every other user anytime they forwarded a message, or cropped a fellow user's photo, or reposted something, lest Facebook later be sued for contributory or vicarious infringement. [2] That's why Facebook has now decided to just have users agree to a waiver in advance.
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[1] Not that you would necessarily win such a lawsuit. There are such things as "implied licenses": If you send email to a public mailing list there's an implication that you mean for it to be public:
http://www.piercelaw.edu/thomasfield/ipbasics/copyright-on-t...
There are also many fair uses. But, especially if I use your writing for some sort of commercial purpose -- as Facebook does, constantly -- you can take me to court and hassle me, because the copyright is still yours. As the Creative Commons website will tell you at great length, under current US copyright law it's actually really hard to throw away the inherent copyright that you hold in your own work, even if you want to throw it away.