As has become clear, “engagement” means public brawling. It generates clicks, and clicks are gold. It also encourages people to sign up for accounts, just so they can jump into the cesspool. Those accounts are quite valuable, as they are fodder for data miners, correlators, and aggregators. Making those accounts free, and easy to create (but not so easy to delete), is a natural.
That’s why so many of these sites wait until their comment fighting pits reach brand-destroying levels of rancor, before they do anything about it; sometimes, too late (remember when SlashDot was the HN? It’s now the place to go, if you want ASCII-art swastikas).
Yeah, I’m cynical.