Governement research centers, such as Inria (in France), require researchers to publish their article for free (on HAL): http://www.inria.fr/en/news/news-from-inria/inria-champions-.... And I think that in this US, governement research must be published as public domain. More generally, the ones who give grants (NSF and so on) could require the research to be published in openaccess as one of the conditions to get the grant.