Authors where paid to do the research and publish their work that produced the paper (that is what the grant was for). PLoS an Open Access publisher pays editors, type sets the work, finds a reviewer and publishes the work for free access on the internet. Reviewers are the ones that generally do not get paid for their work.

Elsevier makes over $3 billion dollars with the closed publication model. They force institutions to pay for bundles of journals they do not want. The Institutions often do not supply access to the general public despite the papers being produced with public money (and despite many of the Institutions being funded by public money).

Paying the cost upfront from the grant increases the availability to the public.