Pure speculation, but if you publish something created by another person without an explicit permission by them, it may open you up for a lawsuit. If some groups required explicit approval by a moderator in order to read the posts, I would take it as they didn't want the content to go public.

So technically, some legal troll could post some copyrighted information, wait for it to be published on Archive, and then sue Archive for copyright infringement and Verizon for assisting it. As a non-profit, Archive will likely get away with just taking it down, but a for-profit Verizon is a wholly different story.