This might just be me, but When going through the low-quality post review, if you click recommend deletion, you get the Add a comment for the author dialog
Now I know that It says "Add a comment" at the top, but what I, as a user, was presented with, was a list of reasons to recommend deletion(like it would be if you voted to close a question)
I didn't know that I was going to leave a canned response on the post, nor did some of the posters know that the response which I've left came from this interface.
I understand that it's a difficult problem to make it both clear, and still have the canned responses in the dialog.
maybe make it clearer that you're leaving a comment if you had a text box next to "Add a comment for the author" for a custom comment of some sort, or maybe if you did away with the titles of the canned comments, and just had the radio button next to the comments themselves, it might be more clear that you're leaving a comment.
What I think is probably the best long-term solution though, is to model the low-quality answer review like the close question review, where instead of leaving a canned comment that might confuse the asker, have some votes for deletion area for answers that reflects the votes to close area. for questions.