They most likely can't but reviewers basically depend on the goodwill of the publishers in order to get pre-release copies for early reviews. If they don't get them they will simply loose out on money (reviewing the game 1-2 weeks after everybody else already did so is kind of pointless) and I'd guess a reviewer would risk getting essentially blacklisted by the whole industry as there is no reason to assume that a reviewer would uphold one NDA but not the other.
Game publishers also tend to advertise their content on said review sites and youtube channels so that would be another income stream gone.