Yes. Also Canada.
> ... the work is entitled to be copyrighted, ...
You're heading in the right direction here, but copyright does not work quite that way in the modern world. No one copyrights (verb) things anymore. Rather, copyright exists in most works upon their creation. So, yes, these are copyrighted works. (And so is your HN comment, by the way.)
> ... and basically you're stealing it and giving it away for free.
The scientists already gave it away to the journal for free. Journals, which provide nothing in the modern world except prestige, now hold the copyright and charge for access. No one is stealing anything, since everyone who had a copy before still has a copy. Yes, the papers are now being given away free to the public, probably against the wishes of the copyright holder (the journal).
> But these scientists didn't do anything wrong by publishing THEIR works (not Public Domain works) behind a pay wall.
Close. The scientists assigned the copyright to the journal, for no payment (but for the prestige of it). The journal is the one publishing behind a paywall, and the journal is the one getting any money collected.
Now, to the real point: "wrong" is not the same as "illegal". Yes, the journal holds the copyright to the paper, and it is legally entitled to charge for access. But what about the moral issue? I funded this work. Me. My taxes paid for it. If you are a U.S. taxpayer, then you paid for it, too. And yet now I'm supposed to pay to see it? There's a problem there.
This would be illegal[1] if these guys were medical researchers funded by the NIH, which has a policy that publications coming out of projects they fund must be freely accessible.[2] It is not illegal here; I think it should be.
And one more thing that many people don't catch: the researchers would probably love to have their papers more accessible. They are not sending papers to pay-walled journals because they like limited access. They are doing it because these are the journals that give them prestigious publications. The journals can trade on their prestige to enforce conditions on the researchers, like limited access. No one likes this except the journals. If NASA made a policy that papers from their projects must be made freely available, then the journals would either have to allow it, or else stop publishing all NASA-based works. Experience strongly suggests that they would do the former.
[EDIT--One more quick note before the editing period closes: It appears the author of the post might not be clear on the distinction between researchers working on NASA projects and researchers doing projects using data that NASA has made available for anyone to use. I am primarily addressing the former situation here. However, what I said about the journals being the only ones with any interest in limited access, applies to academic researchers in general.]
[1] Or maybe not quite illegal, but rather against regulations. Almost but not quite the same thing.