Don't forget that newcomers cannot post comments; you need a minimum reputation to be able to post comments. For people whose reputation (at the time the comment was made!) was low enough, the 'answer instead of comment' is all they can do. There is no cause to chide them.
For people with enough reputation to post comments, then you can suggest that the 'answer' should be a comment - by adding a comment yourself. By the time you're aware of the nuances, you usually have enough reputation to comment.
Sometimes, though, the 'comment' would be so much longer than the 600-character limit that an answer is not an unreasonable way to address the issues you see in another answer. If that were the case, I'd probably leave a comment in the original answer, pointing to the expanded 'comment-as-answer'. If what you say will fit into a comment, a comment is better.
Did you know that the reputations needed for commenting on Meta are different from those on SO? I didn't. On Meta, anyone can comment - the reputation needed is 1. On SO, the reputation needed for leaving comments is 50. That's not hard to reach, but it does take a bit of time.