It does appear to be an unexplained inflammatory comment, or trolling. I think anyone being downvoted or flagged should be given the reason why.

I don't necessarily agree that the commentor bears the burden of arguing that their comment is productive. Sometimes a misguided or misworded comment can lead to productive discourse. Have you ever posted a comment that you thought was wrongly downvoted? I believe each comment is neutral until it has been proven to be productive or proven to be unproductive. I believe this determination can't be made by a single individual, but by a larger group (even then, minorities may be marginalized).