In "University Open-Access Policies as Mandates," Stuart M. Shieber, Director of Harvard's Office for Scholarly Communication, discusses the difference between university open access policies and university open access mandates and whether it matters.
Here's an excerpt:
Try the following thought experiment. Suppose a policy on faculty were established that granted to the university a license in faculty articles but did not explicitly provide for a waiver of the license. Now imagine that a faculty member has an article accepted by a highly prestigious journal that does not allow for author distribution and will not accept an amendment of its copyright transfer policy. Perhaps the author is a junior faculty member soon up for tenure, whose promotion case will be considerably weakened without the publication in question. The author might naturally want to have the license waived even though no waiver is explicitly provided for. The faculty member is likely to storm into the dean’s office, howling about the unconscionable practice of taking rights even when it harms the faculty member. Is the university going to distribute the article anyway against the express wishes of the faculty member? Be serious. The dean says "Fine, we won’t make use of the license for this article." Voilà, a waiver. So much for university rights retention mandates.