It's not surprising this is flagged. If you read the recommendations, discussions of downvoting (and flagging certainly counts) are not welcome because they're circular, uninteresting, and don't contribute positively to the site content.
Why are only ranked users able to flag? My guess is that after someone invests enough time in HN to accumulate 500 karma, that person has some idea of the spirit of what's wanted here. Ads and job postings are certainly not, nor political flames, etc. So a "karma'd" user might spot that pretty quickly. I don't get out my flag flyswatter very often but when I do, it's because I'm trying to help keep the garden clean, so to speak.