I just don't get this. I was involved in a conference committee this year (CS is published in conferences), and as far as I know everyone from the general chair on down was a volunteer. There were some costs like conference space for the program committee meeting that had to be covered via the cost of admission to the conference, but in a journal presumably even those would not exist since you don't need reviewers to meet in person.

As far as I know, the only ongoing costs for the publisher (not immediately recouped via conference admission) would be running their own website and providing downloads. These hardly seem like massive expenses to me.

I don't see any fundamental reason why the model in CS couldn't work for other disciplines even if you don't want to adopt conferences; people are willing to step up and do the work, so why is so much money going to the middle men?