In the computer science world, two of the biggest publishers are the ACM and IEEE professional societies. They both allow authors to post a version on their own webpage (http://www.acm.org/publications/policies/copyright_policy, https://www.ieee.org/publications_standards/publications/rig...). I think this situation is a result of the fact that conferences, not journals, are where CS academics target their latest work. Since the conferences are organized by the professional societies, the professional societies became the biggest publisher. I think this is a good thing, as the professional societies are non-profit organizations that exist for the benefit of their members, and society in general. I hope that they will eventually make their entire digital libraries open.

There are CS journals, but even there the ACM and IEEE are big players.