I think many people post stories for the soul purpose of getting pats on the back; they're just not interested in criticisms, because they take that as a personal attack rather than a critique of the story. Also, the really rubbish stories often, I think, get reviewed by the "author's" friends, who feel obligated to just say nice things to save trouble. If the story is that badly written, other people would likely quickly see how bad it is, cringe, and simply move onto another story.
I leave negative reviews, but only if the author has made it very clear that they really do want a proper critique. Those sort of people are here to actually try and improve their writing, and therefore are happy when people point out mistakes their making. I always try to do it in a positive, encouraging way. Sadly, though, a great many people are only interested in hearing how great they are.