There's a lot of good points here. The forums are a more casual place. We all can misspell a word, or two, or three. I've certainly made my share. I for one take into account to look to see who the poster was when I read their post. I can then over look any mishaps in their written word.
Even the Hosted Authors make mistakes when they post in a topic. Here's an example. When I first joined GA, one of my first posts was in the Editors Corner Forum to introduce myself. One of the authors I work with welcomed me. I had to reply back to him something along the lines of, Gee James, am I going to have to follow you around and correct your spelling errors?
Now as to posted stories, if it has a lot of misspelled word and errors, I quit reading it. I very rarely offer my services as an editor to someone. I have quite a few authors that I work with and some time, they can keep me quite busy.