Governments are actually in a bit of a gray area. On the one hand, a politician can score cheap points with the public by saying "we should all be able to read what our tax dollars pay for!", and on the other hand, the government would ultimately be footing the bill for open access in the short term because their grant money would funnel to publishers by way of pay-to-publish open access fees. So there's significant incentive for the government to figure out an open access solution that involves as little publishing fees as possible. They just don't want to be the ones who pay for it and administrate it.