I think that the hard truth of the movie business is that often the sequel is in the works before the movie even hits the screens. Having some people work on the screenplay could save a lot of money if the movie is a hit - gotta get the next one out as soon as possible, and every day of delay is lost money.
QFT. Keep in mind that they were going to make three "Golden Compass" movies and scrapped the last two after the first one tanked. And there were supposed to be seven "Narnia" movies, and they canned four of them when the second movie underperformed.
Heck, the actors and crew on "Enterprise" were signed up for seven seasons. And that didn't stop the show from being canceled after S4.
All that said, I'm THRILLED that they're already planning for a sequel, because I fully expect the movie to do just fine at the box office, certainly well enough to justify another one.