The "If you have nothing positive to say, don't post it." mentality is a great contributor to the problem.
The opposite mentality, "pick every story apart for the smallest flaws" would result in an even bigger problem, a story site with no authors and consequently no stories.
The authors who post to free story sites are amateurs, they don't have the resources for the kind of professional editing that the dead tree publishers provide.
You can have free stories or you can have literature quality stories. You aren't going to find a site that provides literature quality stories for free.
You get what you pay for.