No one forces the reviewers to sign and agree to the embargo. Sure if they don't agree to it they don't get to see and thus review the movie but no one is forcing their hand. The studio has the right to protect its IP and how information about it is released prior to when it officially is available. Publishers of books and studios do the same thing with DVD releases, the retailers are under restrictions on when they can put the book/item on the shelf.
Now, sure, they can do it early if they wanted to but then they'd suffer consequences most likely being that particular distributer not working with them anymore. It's an all agreed upon process by both parties who know what they are getting into.
If you're a movie reviewer you know going into a private screening you're not going to be allowed to discuss things until a certain time. If you don't like it no one is making you go to the screening. If you DO go to the screening you're making the choice to agree to the contract, terms, and conditions of the screening.
You agree with that practice? Well good for you.