Meh.
It's like with any kind of journalism, you start writing **** up about a company, you won't be able to get any more insider's info, test early releases, interview or have anything to do with the marketing departments, no freebies, no first-hand news, no traffic = you're screwed. On the other hand, it is likely affecting the major gamer news websites/magazines, and has little to no effect on the less known ones.
So you pass it on to your editors, if they do a bad job, they won't get another game to review and someone else will do it for them.
There's really no magic in here, any PR hog like EA will make sure their voice is heard but that doesn't mean they directly "bribe" anyone.
So to your initial question, no, game reviewers aren't "bribe", although there is an indirect influence on them from their peers and most importantly, employers. Even the EA PR guy who was working on the vidya board said there was no money changing hands to his knowledge. And it would be virtually impossible to bribe all reviewers.
EDIT: the top comment mentions money from advertising which is also a huge drive for many websites when it comes to picking favourable reviews.