> If you're being paid to deliver code that means you're being paid to grant certain rights to that code.

For the vast majority of code people are paid to write (certainly the near-entirety of the code I've been paid to write!), the only rights the purchasers actually end up exercising (and therefore actually need granted to them) are the rights to use it and distribute it internally (and maybe to modify it and use/distribute the modifications, but even that ain't a given). The purchasers of that code ain't usually buying it so that they can resell it; they're buying it because it solves an actual problem of theirs, and it would solve that problem regardless of whether or not they're the legal owners of that code.

The reason said purchasers typically want copyright assigned to them is not because of some expectation of resale, but simply to mitigate the risk of some external party denying them the right to use the software in the future. If there is no such party (because the code is in the public domain), then that risk is non-existent. It stops mattering that you're unable to grant any rights upon delivery because no such grant is necessary in the first place.

> And of course depending on the kind of contract it can still be sufficient to deliver code nobody can claim copyright on - but you should definitely check with a lawyer before just assuming things.

In an ideal world we'd all have lawyers on call who can answer all our questions with some assurance of certainty, but in this case it's pretty self-evident that if nobody can claim copyright on something, then that makes it exceedingly difficult for there to be anyone who can claim your use of that thing is illegal.