It seems like you pay for an ad and a positive review will be generated. No cash, no review at all.
No. Plenty of products are reviewed without any ads being sold beforehand, though of course the advertising department then tries to sell ads on the back of the review.
Why no blatantly negative reviews? Well, one reason is that reviewing pays so badly that no-one in their right mind wants to waste time reviewing kit they don't like.
Second, if you do get a camera you don't like, you know that you're going to have to deal with the same company at some time in the future, and you don't want to blow your chances of ever getting anything out of them again. If reviewers bought the kit they reviewed, it would be different, but no-one can afford to do that any more.
Third, very few cameras (or lenses, or anything else) are completely useless, so as suggested elsewhere, you do need to read between the lines to some degree. "Although its performance is not in the same class as a ______, nor is the price" can be a clue. So can such phrases as "ideal for the beginner". Then there are flatly true statements: "This camera is idea for landscape photography, but you probably wouldn't want to use it for nature," usually followed with an explanation such as that it's too slow-handling, or whatever.
Addition: Quite a few products are advertised but not reviewed. This may be because review samples have not been available; or because no-one felt like reviewing it; or because the prodct has been around too long; or because of some other reason. Whichever it is, advertising does
not guaranteed good (or indeed any) reviews.
Cheers,
R.