The thing is, they don't. It's ok to be somewhat biased, even if you're a journalist, but it's not a question of bias in my honest opinion, but more about the maturity of the medium. In movies or music or even technology, journalists take on a more skeptical if not more balanced approach to their reporting. It's easily seen. Look at any Windows or Apple product. Regardless of the marketing and the hype machine of those two massive companies, the journalists are there reporting a balanced on their products, discussing the pros and cons. That's the way it's supposed to be. If a journalist was in bed with Apple, if it seemed like all he did was praise their products and hype their products with slightly veiled criticism, who's going to respect that guy. I sure wont read his reviews, most advlts wouldn't.
A lot of these journalists act like kids from my perspective, not all of them, but a lot of them. So much that I don't even bother reading reviews half the time. Sometimes I think the forum does a much better job of providing a balanced atmosphere of the quality of the product. You can see both sides, but even more, each side has their own consensus on what's good and bad about the game that isn't as far apart as you might expect.
edit: A lot of these so-called "reviewers" aren't even professionals. Typically with any other medium, whether it be technology, music, movies, books, ect, the reviewers have a strong grasp of the history and foundation of the topic they're reviewing and the medium itself. They know where it came from, why it's important, and how it has changed over the years.