Comments should be used for the following (from our help pages):
- Request clarification from the author;
- Leave constructive criticism that guides the author in improving the post;
- Add relevant but minor or transient information to a post (e.g. a link to a related question, or an alert to the author that the
question has been updated).
A comment to an unknown person that's unlikely to ever see the message, there's little value to them. That said, they happen all the time, and are often left untouched.
I'd also recommend keeping comments positive and constructive on the site to be welcoming of others, anything that could be seen as combative by someone else may be flagged for mods to remove. So instead of demanding that someone explain themselves, inquire how the answer could be improved. Still, that's unlikely to ever be answered, so my suggestion in that case is to simply add your upvote since that will more than offset any points lost by someone receiving a downvote.