I think Warner Bros. is in a different place then when they released Superman Returns. Back in 2006, they still had the Harry Potter franchise. Batman Begins had just come out, and The Dark Knight was on the horizon. They also didn't formalize DC Entertainment at that point, and I'm not quite sure they were fully aware of their cinematic plans at that point for DC superheroes.
Flash-forward to 2011, and the final Harry Potter film debuts this month, leaving a huge void that WB needs to fill. On top of that, the lucrative Nolan Batman franchise is ending next year, and Superman is still an unproven commodity. Plus, WB has realized it needs to do something with its DC properties, and they've etched out battle plans and ideas. I think they more actively want to compete with Marvel now, but most importantly, they need franchises to fill the void being left by Harry Potter.
I think Warner Bros. sees DC superheroes as the way to fill that void. I think they envision Green Lantern as an investment. If this movie is deemed a failure, what does that mean for future DC superhero installments a la The Flash, Man of Steel and other characters? WB didn't move forward with a Superman Returns sequel for numerous reasons, one being because they probably didn't really need that particular franchise at that current moment. I mean, the only reason why Man of Steel is moving forward in such a swift and timely fashion is because WB will lose partial rights to the character in 2013 if they don't. I'm fairly certain if WB wasn't in jeopardy of losing the rights the character would still be languishing in development hell.