Extensively using Google Scholar, I've realized that most of the articles belonging to publishers that limit access to material (i.e. ACM, IEEE, Elsevier) also have PDF versions available for free, and these files usually are hosted by universities or other aggregators like ResearchGate.
Is it legal for the university/organization to host a copy of an article, if this has been published on a conference or journal with paid access? Does it matter if the authors of the article belong to that university/organization?