Perhaps it could be because they want other fic community readers to see the nice review and encourage more people to follow the link and read the story? (I guess that's the glass-half-full theory :-)).
...I'd never thought of it that way before. I've seen people saying they're less likely to read a fic with none or little reviews, and where I actually house them, if there's a lot of reviews elsewhere but not on the post, well... "the lurkers support me in email! :O!"
Thanks for the input though. Since I've seen people saying if it's too much work to comment (like writing up an email) to do so, they're less likely to in the first place, it seemed to me to be more work to double back and comment to the original entry so that was also a source of my confusion.