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It is incorrect to say that authors publish freely. A general rule of thumb is that journals run by societies (J. Electrochem. Soc. for example) charge pages fees to authors and operate at break-even or at a loss due to the low or zero subscription fees for members (the ECS membership fee of $98/yr includes free online-only access to J. Electrochem. Soc.). Journals that are run by for-profits or have high subscription rates do not charge page fees for authors.
Also, the peer-review process is not free for the journal. The journal needs to identify reviewers, send out manuscripts, hound reviewers to send their reviews in, deal with conflicting reviews, get revisions from authors, etc. And that doesn't include typesetting, publishing, etc.
In the case of journals that do not charge page fees to authors, the government has spent no money to publish in those journals. These papers are distinct from the progress and final reports that contractors to the government (companies and University and National Lab researchers) provide as a requirement of their receipt of government funds. As such, the public does not have a clearly identifiable right for free access to the journal papers, but should, and in most cases do, have access to the final reports provided to the government.
These two types of publications serve two distinct purposes. The reports to the contracting agency provide a more broad-based, usually application-oriented, description of the work. The scientific papers provide more detail for those people/parties working in the field. Those who are interested in these details can purchase subscriptions to the journals.
As an aside, in 15+ years of research I have never come across a journal that provides free access 6 months after publishing. This parctice is apparently limited to the bio and med journals due to NIH requirements. But, almost all of the journals available online have free access to article abstracts.