I believe much of it is a matter of timing. When a story is up anonymously, the author has her hands tied. She cannot change the typos or rewrite awkward passages, or respond to defend her writing or even agree with criticism. I encourage concrit actively with anything I write, but would think WTF if told about typos on an anon fic. If the real reason for the concrit is to make it a better story, either posting it after the reveal or leaving a comment like 'I have a few bits of concrit--remind me to tell you later' would be more helpful.