This is something that most of us are guilty of, including myself. Authors that I edit for, or beta read for, get comments sent back with the chapter. I don't do it enough for authors that I just read.
This isn't the first time an author has done something like this. My best advise is, let an author know what you think of their stories/chapters by particpating in their forum, leaving a comment with the story, or sending them a PM or email.
When you PM/Email the author, just don't say you like a chapter, let them know why you liked it. If you feel the need to criticise then do it constructively. If you do it right, you may just make yourself a friend. I have an author/friend that we talked about the story enough, if I had a question, he'd challenge me to find the answer and report back to him. A plus to all this is that we've actually met in person twice over the past few years.