I made an assumption and was called on it on Liblicense. I was under the impression that to be published you ( or someone from your place of work if it's work-for-hire like at mpow) had to have signed a copyright agreement. Turns out that over time, some publishers have been a little slipshod with this. They didn't follow up or they made some statement that if the author didn't sign, the transfer was assumed or claimed or whatever. Even if they got the agreement, they might not still have it.
Huh. According to Laura Quilter on LibLicense, in the US there has to be a signed agreement. So... makes you wonder... could these people who never signed agreements claim their own copyright back? Post their articles wherever they want? Or maybe it would still have to be the content stripped of all the publisher specific formatting and logo?
P.S. - read what you sign! consider the SPARC or other addendum.