The downvotes are probably because of the "Please don't comment about the voting on comments. It never does any good, and it makes boring reading." guideline in the HN guidelines linked at the bottom of the page, which HN readers seem to interpret as including comments discussing flagging on submissions as well.
Nobody seems to be permitted to explain the flagging either because of the "Please don't complain that a submission is inappropriate. If a story is spam or off-topic, flag it. Don't feed egregious comments by replying; flag them instead. If you flag, please don't also comment that you did." guideline. Anybody who does post to explain their reasons for flagging seems to often have their comment downvoted and flagged.
I'm not arguing for or against the guidelines, just pointing them out.
