Why is it legal to set rules for people to review your product?

[Words Of Wisdom]

Hapimeses said:
In short: the intellectual property (IP) used in the review is privately owned. If you want review code early -- i.e.: you want access to their privately owned IP before the rest of the market -- you may be requested to sign the equivalent of an non-disclosure agreement, limiting your ability to freely talk about their IP. However, once that IP is released freely on the market, such limitations come to an end for obvious reasons.

So, if you want a review potentially filled with spoliers, or discussing aspects of the game that the publisher does not wished to be discussed, wait until after the game has had its official release.

That's about it. Nothing especially sinister, although it does pass control to the IP owners rather than the reviewers. And given most reviewers want their reviews out early, they sometimes have to sacrfice their free speech in the short term to achieve this.

This is the big catch.

Reviewers aren't entitled to early copies of anything.  Those copies are provided by developers/publishers at their discretion.  If the reviewer wants an early copy of a game, they must abide by the stipulations of the provider.  It's essentially a contract between reviewers and providers.

It serves both parties too.  Providers can dictate terms of the review and reviewers can get reviews out before their competition.