It's to keep reactions of it purely real and unbiased. Actually, it's to manage expectations of the movie. They don't want reviews swaying public opinion of the movie.
A review before the movie comes out can make or break a box office weekend, especially this close to the big blockbuster movies that are coming this year (the couple that ARE coming, at least). You don't want to be tanked before you even get started. Honestly, it's the new form of censorship.
The same holds true for gaming reviews. If you don't give Call of Duty at least a 9 out of 10 in your review, then Infinity Ward and Activision have you blacklisted as a reviewer and source. The same holds true for Battlefield and EA. Ask Jim Sterling. He's the living proof of this.