The co-founder of open-access publisher PLoS, Mike Eisen (who has guest-posted here) recently took his legal life in his hands and posted articles in Science magazine outside the paywall.

His rationale: The Mars Curiosity rover research was government-funded and NASA can't can't turn over copyright to Science because copyright law explicitly says that all works of the federal government are not subject to copyright.

The nuance may be that the authors of the paper are not NASA employees and that even NASA employees certainly can grant copyright - their work -  to corporations. It's happened throughout the history of NASA. Only the raw data itself may be public domain.

NASA paywalls first papers arising from Curiosity rover, I am setting them free By Prof. Michael Eisen