They are professional organizations, not just publishers. As professional organizations, their primary goal is to support their members and the betterment of the field. They may not always have paywalls; unlike Elsivier, their organization exists for purposes other than getting money from publications.

The co-chairs of the ACM Publishing Board recently wrote an editorial, "Positioning ACM for an Open Access Future": http://cacm.acm.org/magazines/2013/2/160170-positioning-acm-...

It's not enough, but it's a start, and I'm hopeful. I am a member of both the ACM and IEEE, and I want both of these organizations to move to a fully open access model.