If a plot twist or finale is bungled, being forbidden to write about it means being forbidden to write an effective review.
The effort to supposedly discuss movies without details very frequently makes reviews worthless, hardly more than a verbose way of saying year or nay.
If the complaints in this thread are any indication, reviewers haven't been muzzled from spoiling plot twists or describing important details when it comes to The Dark Knight Rises
. Reviewers can, and do, write whatever they want. If they don't regularly describe the conclusions of movies in detail it's unlikely it has anything to do with studio enforced agreements -- rather, it has to do with the reality that most people who read reviews do so in order to decide whether or not to see a movie, not to spoil the ending of that movie.
So you believe that the bosses are easily satisfied with the service they get? And don't think they deserve even better, done exactly the way they want?
No need to patronize. You said the "reviewing system is amazingly corrupted already." The fact that a major studio invented a film critic suggests that the major studios don't have the control over film critics that your description implies; the fact that studios regularly forgo press screenings for movies they think are bad further supports this.
A further detail -- although I'm sure it differs for tentpoles, I've been to about a dozen press screenings this year for independent and mid-range films and never been asked to sign a confidentiality agreement. I have been asked, verbally, to hold any reviews until a certain date at a few screenings.