It may not even be those with strong opinions on the topic that are flagging this. I think some may flag topics they see as potentially inflammatory as a means to avoid flamewars, uncivil discourse, etc.
"Please don't post insinuations about astroturfing, shilling, bots, brigading, foreign agents and the like. It degrades discussion and is usually mistaken. If you're worried about abuse, email hn@ycombinator.com and we'll look at the data."