Besides that, I think it's a somewhat small attempt to hold people
accountable for the batshit and/or contemptable things they say to
others online.
If that has even a grain of truth, then I rebel against it further. If you're going to open commentary on your content, and then try to control the commentary through identity politics, any incentive to provide commentary (for myself, anyway) is entirely stripped. By controlling the nature of the commentary, by assosciating one's name with one's comment (which anyone who has grown up with the internet as both a tool and a weapon will tell you is a bad, bad idea), the ability to articulate freely one's opinion without sanction is stripped. The internet is an anarchy, a generally faceless, lurkery mass of people, and by trying to impose some level of unchecked "accountability" through Facebook identity politics the quality of discussion is undoubtedly lowered.
Sure, people say batshit crazy things online. But just as many people post articulate, intelligent thoughts associated with non-invasive usernames that do not compromise their personal identiy or personal lives. Slate undoubtedly has a firm of lawyers to protect the rights of their columnists. If they choose to retain commentary, then we, the psuedonomic mass of lurkers who respond here, freely, voluntarily, must retain the one tool we have in the face of similar loss of rights: the right to use an alias to express our opinion.