For one, a plumber directly charges the end user x amount of money based off of expertise & reputation.
Where as a reviewer gives reviews away FOR FREE just hoping that someone will click on the link the review and they will generate a little add revenue in the process.
Completely different animals.
That is the business model for the lone blogger, but certainly not the one for most reviewers. The freelancer will sell his pieces to different publications, the stringer will usually work with a single publication but be paid on a per character basis. I suspect the guys at The Escapist, Kotaku, etc have a set salary.
I am not saying that the comparison with the plumber is a good one though. An aspect that has been notably forgotten so far is that, to do his job, a reviewer needs to have the products before their street date (and in the case of paper magazines, a long time before), so they cannot buy them in stores. You could argue that publishers could bill the reviewers for the review copies they send them, but that would be a bit odd and would go against their interest as they want their product to be reviewed.
Note that this is not specific to video games at all. I am not sure how it goes when it comes to electronics or cars, but movies, music, literature, comics, DVDs and even performance arts work the same way. The only exception I can think of is reviewers for restaurants, because how well the consumer is treated is part of the review.
As for the optimism in previews, I think it's a multi-faceted problem. Firstly, the reviewer is confronted with a work in progress, so he has to give the benefit of the doubt to the dev' (previews often note the existence of problems that should 'hopefully be fixed before launch') and secondly, who would want to read pessimistic previews? If a title is obviously shaping up to be a total train wreck, the reviewer has to say it, but the overall tone of a preview section should be optimistic (something that can be done without lies and manipulation, but simply by picking the titles the reviewers are optimistic about). Another aspect is that the average reviewer is usually more jaded about games than the average reader (when I read the Kotaku review for Killzone and the guy is like 'we saw this and that it a dozen fps before' he forgets to mention that it can still be pretty fresh for people who have not played this dozen of fps before).
