Public domain is largely a US government concept, I think the was the main reason that sqlite was put "in the public domain" it started and was initially payed for as an internal database for some military contract. So when Dr. H wanted to release it to the public he did it in the normal US government method which was public domain.
Would you legally be able to use nasa papers or programs? these are also in the public domain.
A short essay on public domain, because I am feeling a bit philosophic this morning.
As far as I know public domain is a concept that originated in the US when it was a new country(probably wrong and US centric). The question was who owns works created by a government "of the people", the answer was "the people" so it was formalized that US government works were not protected by copyright[note 1] and were in the "public domain" at first this was just laws, and when government research organizations(nasa etc..) were founded their works, the research they did, was also "public domain"
It really is an open question whether private citizens can also declaim any copyright on a work, declare it "for the public good" and put it in the public domain. Are you allowed to give up your rights?
1. The US government is able to protect it's works through mechanisms other than copyright, think classification levels. top secret etc..
As for NASA papers and such... I have no idea. If I were running a company I'd be able to ask a lawyer; as it stands, I'm taking the risk that the SQLite authors don't sue.
Similar to physical property: being able to sell it or to give it to the public is an important right. (Okay, you probably can't give your American land to everyone in the world, but only for practical reasons, and not for some fundamental reason like that would be "giving away your rights". No, that would be the opposite: your land, you do whatever you want with it.)
We're allowed to use public domain works, but we cannot declaim the work. Your copyright has a fixed term, which outlives you. Fin. You cannot surrender a right - as that has caused abuse issues in the past.
No right can be surrendered, but some may be suspended temporarily by a narrow set of laws I wish was even narrower.
Unfortunately... This does mean we can't work on Public Domain projects (legally), as we keep our copyright to our changes. Though practically, that has only been contested once in the history of the nation. The intent of the individual tends to be respected by our Common Law system.
i.e. PD is a signal the creator wants a free and boundless reuse of the product, even if it isn't strictly in the public domain.
However, on the face of it I have no right to use the software without buying a license.
The only part of the US public domain that won't fly in many countries is the waiver of moral rights (mainly the right to attribution). Since my country does not allow this waiver, the authors might be able to sue me in my jurisdiction if I started claiming to be the author of the code, which would have been ok in the US. But the voluntary non-exclusive transfer of usage, modification and distribution rights is surely allowed in every jurisdiction, so there is no reason that that part of the dedication would not be valid.
And if there exists a jurisdiction where the transfer of material rights isn't allowed at all, selling a license also wouldn't fix anything. Unless there's a jurisdiction out there somewhere that allows the transfer only in exchange for money or in a one-on-one contract, but then open source licenses also wouldn't apply there and I feel like I would've heard about it as a fun fact somewhere on the Internet by now...