My answer to https://cs.stackexchange.com/q/13495/755 was converted to a comment. Why?
I believe my answer was an acceptable answer. The question was "How do you show..." and my answer described how: it said "I would suggest that you start by... it'll probably even give you some examples ... which you can apply to [this problem] ...". My answer was not a request for clarification; it was a bona fide answer that described how to solve the poster's problem.
I read through the official criteria for when an answer should be converted to a comment, and none of those apply (as far as I can see). And there are reasons why answers should remain, well, answers: e.g., questions without an answer get treated differently, comments aren't searchable, others cannot edit a comment to improve it, others cannot downvote a comment, the original poster cannot mark a comment as the accepted answer, etc.
Obviously, this case is very minor: it's just one question, and not even an important one, so big deal. I don't really care about this one case. But I do care about the general policy; I think being overly aggressive about converting answers to comments is not so great for the site.
So, why was this converted to a comment? Was it just an oversight? Or is there some general policy that I've missed that explains why this conversion was the right thing to do in this situation?
Edit: The moderators subsequently closed the question. I do not have any objection to moderators converting my answer to a comment, if they're going to close the question as well. I think this topic is now moot. Thanks to Gilles for his detailed thoughts and explanations.