It's because publishers are concerned that if research is available to the public immediately, institutions will stop paying extortionate fees for journal subscriptions. They have considerable lobbying efforts and several Congressmen willing to change legislation to their benefit.
"The Administration also recognizes that publishers provide valuable services, including the coordination of peer review, that are essential for ensuring the high quality and integrity of many scholarly publications. It is critical that these services continue to be made available."
If these are valuable services, then customers will pay for them; no need to have a monopoly on access to published research.
(as a paper author and reviewer, I don't doubt they are valuable services, but given that reviewers do the work for free, and the research has been paid for, I don't see why the access should be limited.)
You can either charge the author customer, which is what open access does, or charge the reader customer, which is what non open access does. Insisting that the reader customer not pay just increases the costs elsewhere.
If the benefits of open access are considered to be worth more than the amount lost by not charging for access, then you need to fund that from elsewhere. If every grant costs an extra $2,000, there will be less grants, it's just math.
(Previously I was told that the cost is minuscule compared to the cost of the study itself in general, in which it won't have a large effect. But I haven't seen data.)
If we decide we want the government to also pay the cost of publication, as opposed to having readers pay, great. But it might be expensive, again, data would be helpful.
Blanket statements like "taxpayers paid for it, therefore it should be open" ignore the reality that publishers subsidize publication precisely because they charge for access.
Why wait 12 months? Why can't published research be available immediately?
Does anyone know or have a citation for the rationale behind this?