Okay, they don't want to read their slush, but they're hoping other people will be eager to?
What Victoria Strauss doesn't mention (although a commenter points it out) is that OWW spawned many successful writers—Karin Lowachee, Charles Coleman Finlay, and Elizabeth Bear for starters, but they didn't get their start from Del Rey. OWW functions fine as a workshop where writers help each other. And I think that paradigm works well in general, as non-publisher sites like Critters prove.
What publisher-run sites want to accomplish is to pan for gold from slush at minimal cost, but so far, they have each gone about it by creating a site that attracts writers but not readers. The feedback on a writer-vs.-writer site can be a lot like the feedback on Survivor. Voracious readers who can't afford to buy the books they want aren't sitting at their PCs looking to read unpublished books online. They're at the library, checking out books.
Will Authonomy break the mold? It will be interesting to see.