Well, the legal thinking behind such TOS agreements is simple: they have no control over where your posts show up (google, printed in the NYTimes, etc) so have you assign all rights so you can't sue them because, say, a business article in the NYTimes quotes you as saying your boss sucks or whatever.
The paranoid assume that the motive is to sift through the content andn to use it as the basis of a multi-billion dollar hollywood smash hit without compensating the user/authors of the site content :)
They'll probably remove it, just like MS and others have done, if enough people scream but I have no doubt the motivation is a legal cover-thy-ass-from-all-threats-real-and-imaged frame of mind, not some sneaky and clever way to get you and I to add commercially valuable content for free.
