Because the article title is misleading. Purely publicly-funded research is freely available (e.g. NASA reports). What's at issue are e.g. papers written pursuant to research that is the subject of public funds, but by research teams who are otherwise employed by a university or research lab. In that case, the resulting paper isn't purely a product of public funding, so it's not obvious it should automatically be freely accessible.
Also, these papers are generally the result of editing/publication processes by journals. The government isn't paying for those papers to be edited and published.
To some extent, they are though. Most peer reviewers are not paid for their time; instead, they are largely federally-funded academics. The journals have an incredible racket: they don't pay for their raw inputs, and they get a lot of their value-added for free as well. Since there's been a large shift away from the dead-tree versions being critical for researchers, the cost of transporting bits is a lot lower than stacks of paper.
Also, these papers are generally the result of editing/publication processes by journals. The government isn't paying for those papers to be edited and published.