Well, they can't. Eventually. But all of the people who Nvidia have sampled on this launch (and so covering all reviews that'll be available when the embargo expires)
have signed an extremely restrictive multi-year NDA that puts them entirely under Nvidia's boot. Even AIBs aren't allowed to sample cards to people who aren't on Nvidia's approved list, so the only way to bypass it will be buying a card along with everybody else. So they'll do what they're told or face the wrath of Nvidia's legal department, one would imagine, given that the NDA is extremely broad and open to interpretation (meaning Nvidia can claim pretty much anything to be in breach of it). The reviews from people who aren't beholden to Nvidia likely won't arrive for at least a few days to a week after launch, depending on how quickly they can conduct their testing.