Well, when the GL page on IMDB was IMDB-Pro only, it was indicated that Legendary was a co-producer then. Besides, having them co-finance and co-produce GL makes it easier on WB, since they won't have to invest as much money and P&A on an untested property. (WB reduced their losses on Watchmen considerably when Legendary and Paramount co-funded it too.)
And since the success of TDK, Warners would be foolish not to have Legendary on another potentially big DC comic-book pic. It's a fresh new idea, it has some pop culture awareness, and if done right, could be WB/DC's answer to IM.