Re: "it pays out something far more valuable than money: attention"
Yes, it's one of the standard internet cons, much like the idea of Interns in the wider business world.
You don't have to look very hard to see this scam being used, and outfits such as The Huffington Post even try it on award-winning authors, people who can hardly be said to be short of exposure.
And the whole internet is built on copyright scams. A quarter century ago, companies were claiming a perpetual and unlimited grant of copyright by you, for anything you posted. In those days it was AOL. The language has changed slightly (under US Law, up until the 1970s, you couldn't license copyright, only sell it, and internet companies have now realised that), and the core problem remains, but there still want an unlimited, perpetual, grant of license.
The core problem is that the internet can only work by making copies of content, even transient copies in a router, and even as you read this, your browser has made a copy. So the internet companies need to make some sort of copyright claim. And it it practical to remove every copy of a work from a well-run back-up system? But we seem to still be getting trampled on by legal boiler-plate written by somebody who didn't understand the internet.
And so we get the offer of "exposure", and if you post a picture of a news event to the net, it can get used without permission or payment by the news media, and if we give their copyrights the same respect they send the lawyers in.