It all comes down to every PM's favorite metric: engagement . When users get into a heated flamewar, that's high-engagement. So you want to do everything you can to ensure that the most possible users get sucked into flamewars. I feel like this should be banned, but it's unclear what the best approach would be. Mandating chronological feed order?
Each of your friend's comments would still give FB a "chronological" reason to bump this post to the top of your feed, enticing you to get into the mix and increase the viral engagement of the original "bad stuff".