I'd actually say it's more a matter of the professionals not being allowed to be completely subjective, rather than objective. A critique is a completely subjective thing, but I think the professionals are pressured to be more "objective" and attempt to predict how the industry and consumer base will respond to a game, rather than be truthful and state the conclusions that they genuinely came to. That's why you see something like Final Fantasy always get high scores and praise. Big-name games like that are expected to be good, and the publications not only don't want to tick off the advertisers but also don't want to go against the mainstream opinion of their readers. The readers are often young and petty, and if you give a score 1/10th lower than what they feel is the "correct" score, that can be enough to lose a subscription. General rule is that if a big-name product is released, it score has to be inflated, while the low profile stuff can get honest reviews (although those can swing the other way in which if they think the mainstream won't give a crap, they can't seem "outlandish" by giving a particular game a really high score; advertisers might also take issue if a low budget, no-name game gets more praise than something they've dumped money into).

As for previews vs. reviews, I don't think negativity in previews is really call for. But neither is gushing. Previews and reviews are two entirely different animals. A review is a critique, designed for no purpose but to assess the quality, while a preview is more about showcasing data and images/videos. A preview should be pretty neutral, but I think some level of optimism is fine, since Lord knows we have enough jaded, snarky professionals. If the person writing the preview hasn't even played the game, he/she really is in no place to criticize it, and if they've only briefly tried a demo, that's not enough to come to any serious conclusions either. Any criticism, like if the controls aren't so hot for example, should be accompanied by about how far along the game is.