The government, in its many forms, funds a large portion of academic science. This has given many people the idea that taxpayers should have access to the output of that science. But in many cases the output is a publication in a subscription journal, which non-subscribers don't have digital access to. Somebody up and said, "Hey, we paid for that research, we want access to it." So NIH has reached understandings with many of the corporations that publish scientific journals saying that if an author was funded by the NIH while working on any part of a paper, the journal has to make that article free to the public, even if the rest of the journal is subscription only.
This works out great for me. My fellowship is through National Institute on Aging, part of NIH. So any journal article I publish while I am on fellowship, or based on data I gathered while on fellowship, can't be hidden from the eyes of non-subscribers.