Reviews are a way for us to share our discoveries. When a friend stops by, you put on your favorite new CDs and talk about how excited you are about the album. You don't say, here's some stuff that I want to show you is bad; let's spend our time together listening to bad music. The only time you might do that is to show how something you thought would be good disappointed you.
I don't think it is wise to write a review unless one has spent time with the item, and we tend to spend our time on components that we like. If I hear something in a store and find it disagreeable, I'd just move on to something else. I'm not going to spend my time evaluating it or comparing it to other products, much less buy it and live with it to know it inside and out.
I think that magazines seem to print positive reviews because the components they review are good. Companies improve their products over time and utilize the best of what's currently out there. Who would bother to make a $10,000 pair of speakers that was bad? A company won't stay in business long if they make stuff that is poor, and they sure wouldn't submit it to a magazine for review if they didn't think it would fare well. I mean, if you think ad money is buying good reviews, then name me some components that got good reviews that shouldn't have.
Also, think about how you read these magazines. I find myself diving into reviews of the gear that I've heard a good buzz about, so those reviews will generally be positive. Why would I care to read a review of something that I don't think I'm going to like anyway? Over time, your impression becomes that a magazine always prints positive reviews. I have seen many bad reviews in Stereophile. I remember one even flat out saying that the cheaper preamp reviewed in the issue was superior in every way -- guess which component's review I initially bought that issue to read?
As a reader of stereo magazines, I don't necessarily read them looking for good or bad reviews. I read them to find out what they think of the new line of, say, Wilson speakers (do they improve on the older line? etc.). I wouldn't want to read Rolling Stone magazine if they devoted space to bad music. I read it to find out about up-and-coming good stuff.